Adding upstream version 1.16.0.upstream/1.16.0

Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>

15 files changed, 1254 insertions, 455 deletions

diff --git a/database/engine/README.md b/database/engine/README.md
index 28a2528cb..adc69ffd7 100644
--- a/database/engine/README.md
+++ b/database/engine/README.md
@@ -96,9 +96,9 @@ There are explicit memory requirements **per** DB engine **instance**, meaning *
 
 - `page cache size` must be at least `#dimensions-being-collected x 4096 x 2` bytes.
 
-- an additional `#pages-on-disk x 4096 x 0.06` bytes of RAM are allocated for metadata.
+- an additional `#pages-on-disk x 4096 x 0.03` bytes of RAM are allocated for metadata.
 
-    - roughly speaking this is 6% of the uncompressed disk space taken by the DB files.
+    - roughly speaking this is 3% of the uncompressed disk space taken by the DB files.
 
     - for very highly compressible data (compression ratio > 90%) this RAM overhead
       is comparable to the disk space footprint.
@@ -106,4 +106,40 @@ There are explicit memory requirements **per** DB engine **instance**, meaning *
 An important observation is that RAM usage depends on both the `page cache size` and the 
 `dbengine disk space` options. 
 
+## File descriptor requirements
+
+The Database Engine may keep a **significant** amount of files open per instance (e.g. per streaming
+slave or master server). When configuring your system you should make sure there are at least 50
+file descriptors available per `dbengine` instance.
+
+Netdata allocates 25% of the available file descriptors to its Database Engine instances. This means that only 25%
+of the file descriptors that are available to the Netdata service are accessible by dbengine instances.
+You should take that into account when configuring your service
+or system-wide file descriptor limits. You can roughly estimate that the netdata service needs 2048 file
+descriptors for every 10 streaming slave hosts when streaming is configured to use `memory mode = dbengine`.
+
+If for example one wants to allocate 65536 file descriptors to the netdata service on a systemd system
+one needs to override the netdata service by running `sudo systemctl edit netdata` and creating a
+file with contents:
+
+```
+[Service]
+LimitNOFILE=65536
+```
+
+For other types of services one can add the line:
+```
+ulimit -n 65536
+```
+at the beginning of the service file. Alternatively you can change the system-wide limits of the kernel by changing `/etc/sysctl.conf`. For linux that would be:
+```
+fs.file-max = 65536
+```
+In FreeBSD and OS X you change the lines like this:
+```
+kern.maxfilesperproc=65536
+kern.maxfiles=65536
+```
+You can apply the settings by running `sysctl -p` or by rebooting.
+
 [![analytics](https://www.google-analytics.com/collect?v=1&aip=1&t=pageview&_s=1&ds=github&dr=https%3A%2F%2Fgithub.com%2Fnetdata%2Fnetdata&dl=https%3A%2F%2Fmy-netdata.io%2Fgithub%2Fdatabase%2Fengine%2FREADME&_u=MAC~&cid=5792dfd7-8dc4-476b-af31-da2fdb9f93d2&tid=UA-64295674-3)]()
diff --git a/database/engine/datafile.c b/database/engine/datafile.c
index 2d17d05e4..8ef4ed599 100644
--- a/database/engine/datafile.c
+++ b/database/engine/datafile.c
@@ -49,44 +49,69 @@ static void datafile_init(struct rrdengine_datafile *datafile, struct rrdengine_
     datafile->ctx = ctx;
 }
 
-static void generate_datafilepath(struct rrdengine_datafile *datafile, char *str, size_t maxlen)
+void generate_datafilepath(struct rrdengine_datafile *datafile, char *str, size_t maxlen)
 {
     (void) snprintf(str, maxlen, "%s/" DATAFILE_PREFIX RRDENG_FILE_NUMBER_PRINT_TMPL DATAFILE_EXTENSION,
                     datafile->ctx->dbfiles_path, datafile->tier, datafile->fileno);
 }
 
+int close_data_file(struct rrdengine_datafile *datafile)
+{
+    struct rrdengine_instance *ctx = datafile->ctx;
+    uv_fs_t req;
+    int ret;
+    char path[RRDENG_PATH_MAX];
+
+    generate_datafilepath(datafile, path, sizeof(path));
+
+    ret = uv_fs_close(NULL, &req, datafile->file, NULL);
+    if (ret < 0) {
+        error("uv_fs_close(%s): %s", path, uv_strerror(ret));
+        ++ctx->stats.fs_errors;
+        rrd_stat_atomic_add(&global_fs_errors, 1);
+    }
+    uv_fs_req_cleanup(&req);
+
+    return ret;
+}
+
+
 int destroy_data_file(struct rrdengine_datafile *datafile)
 {
     struct rrdengine_instance *ctx = datafile->ctx;
     uv_fs_t req;
-    int ret, fd;
-    char path[1024];
+    int ret;
+    char path[RRDENG_PATH_MAX];
+
+    generate_datafilepath(datafile, path, sizeof(path));
 
     ret = uv_fs_ftruncate(NULL, &req, datafile->file, 0, NULL);
     if (ret < 0) {
-        fatal("uv_fs_ftruncate: %s", uv_strerror(ret));
+        error("uv_fs_ftruncate(%s): %s", path, uv_strerror(ret));
+        ++ctx->stats.fs_errors;
+        rrd_stat_atomic_add(&global_fs_errors, 1);
     }
-    assert(0 == req.result);
     uv_fs_req_cleanup(&req);
 
     ret = uv_fs_close(NULL, &req, datafile->file, NULL);
     if (ret < 0) {
-        fatal("uv_fs_close: %s", uv_strerror(ret));
+        error("uv_fs_close(%s): %s", path, uv_strerror(ret));
+        ++ctx->stats.fs_errors;
+        rrd_stat_atomic_add(&global_fs_errors, 1);
     }
-    assert(0 == req.result);
     uv_fs_req_cleanup(&req);
 
-    generate_datafilepath(datafile, path, sizeof(path));
-    fd = uv_fs_unlink(NULL, &req, path, NULL);
-    if (fd < 0) {
-        fatal("uv_fs_fsunlink: %s", uv_strerror(fd));
+    ret = uv_fs_unlink(NULL, &req, path, NULL);
+    if (ret < 0) {
+        error("uv_fs_fsunlink(%s): %s", path, uv_strerror(ret));
+        ++ctx->stats.fs_errors;
+        rrd_stat_atomic_add(&global_fs_errors, 1);
     }
-    assert(0 == req.result);
     uv_fs_req_cleanup(&req);
 
     ++ctx->stats.datafile_deletions;
 
-    return 0;
+    return ret;
 }
 
 int create_data_file(struct rrdengine_datafile *datafile)
@@ -97,21 +122,17 @@ int create_data_file(struct rrdengine_datafile *datafile)
     int ret, fd;
     struct rrdeng_df_sb *superblock;
     uv_buf_t iov;
-    char path[1024];
+    char path[RRDENG_PATH_MAX];
 
     generate_datafilepath(datafile, path, sizeof(path));
-    fd = uv_fs_open(NULL, &req, path, O_DIRECT | O_CREAT | O_RDWR | O_TRUNC,
-                    S_IRUSR | S_IWUSR, NULL);
+    fd = open_file_direct_io(path, O_CREAT | O_RDWR | O_TRUNC, &file);
     if (fd < 0) {
-        fatal("uv_fs_fsopen: %s", uv_strerror(fd));
+        ++ctx->stats.fs_errors;
+        rrd_stat_atomic_add(&global_fs_errors, 1);
+        return fd;
     }
-    assert(req.result >= 0);
-    file = req.result;
-    uv_fs_req_cleanup(&req);
-#ifdef __APPLE__
-    info("Disabling OS X caching for file \"%s\".", path);
-    fcntl(fd, F_NOCACHE, 1);
-#endif
+    datafile->file = file;
+    ++ctx->stats.datafile_creations;
 
     ret = posix_memalign((void *)&superblock, RRDFILE_ALIGNMENT, sizeof(*superblock));
     if (unlikely(ret)) {
@@ -125,19 +146,21 @@ int create_data_file(struct rrdengine_datafile *datafile)
 
     ret = uv_fs_write(NULL, &req, file, &iov, 1, 0, NULL);
     if (ret < 0) {
-        fatal("uv_fs_write: %s", uv_strerror(ret));
-    }
-    if (req.result < 0) {
-        fatal("uv_fs_write: %s", uv_strerror((int)req.result));
+        assert(req.result < 0);
+        error("uv_fs_write: %s", uv_strerror(ret));
+        ++ctx->stats.io_errors;
+        rrd_stat_atomic_add(&global_io_errors, 1);
     }
     uv_fs_req_cleanup(&req);
     free(superblock);
+    if (ret < 0) {
+        destroy_data_file(datafile);
+        return ret;
+    }
 
-    datafile->file = file;
     datafile->pos = sizeof(*superblock);
     ctx->stats.io_write_bytes += sizeof(*superblock);
     ++ctx->stats.io_write_requests;
-    ++ctx->stats.datafile_creations;
 
     return 0;
 }
@@ -182,25 +205,17 @@ static int load_data_file(struct rrdengine_datafile *datafile)
     struct rrdengine_instance *ctx = datafile->ctx;
     uv_fs_t req;
     uv_file file;
-    int ret, fd;
+    int ret, fd, error;
     uint64_t file_size;
-    char path[1024];
+    char path[RRDENG_PATH_MAX];
 
     generate_datafilepath(datafile, path, sizeof(path));
-    fd = uv_fs_open(NULL, &req, path, O_DIRECT | O_RDWR, S_IRUSR | S_IWUSR, NULL);
+    fd = open_file_direct_io(path, O_RDWR, &file);
     if (fd < 0) {
-        /* if (UV_ENOENT != fd) */
-        error("uv_fs_fsopen: %s", uv_strerror(fd));
-        uv_fs_req_cleanup(&req);
+        ++ctx->stats.fs_errors;
+        rrd_stat_atomic_add(&global_fs_errors, 1);
         return fd;
     }
-    assert(req.result >= 0);
-    file = req.result;
-    uv_fs_req_cleanup(&req);
-#ifdef __APPLE__
-    info("Disabling OS X caching for file \"%s\".", path);
-    fcntl(fd, F_NOCACHE, 1);
-#endif
     info("Initializing data file \"%s\".", path);
 
     ret = check_file_properties(file, &file_size, sizeof(struct rrdeng_df_sb));
@@ -221,15 +236,21 @@ static int load_data_file(struct rrdengine_datafile *datafile)
     return 0;
 
     error:
-    (void) uv_fs_close(NULL, &req, file, NULL);
+    error = ret;
+    ret = uv_fs_close(NULL, &req, file, NULL);
+    if (ret < 0) {
+        error("uv_fs_close(%s): %s", path, uv_strerror(ret));
+        ++ctx->stats.fs_errors;
+        rrd_stat_atomic_add(&global_fs_errors, 1);
+    }
     uv_fs_req_cleanup(&req);
-    return ret;
+    return error;
 }
 
 static int scan_data_files_cmp(const void *a, const void *b)
 {
     struct rrdengine_datafile *file1, *file2;
-    char path1[1024], path2[1024];
+    char path1[RRDENG_PATH_MAX], path2[RRDENG_PATH_MAX];
 
     file1 = *(struct rrdengine_datafile **)a;
     file2 = *(struct rrdengine_datafile **)b;
@@ -238,7 +259,7 @@ static int scan_data_files_cmp(const void *a, const void *b)
     return strcmp(path1, path2);
 }
 
-/* Returns number of datafiles that were loaded */
+/* Returns number of datafiles that were loaded or < 0 on error */
 static int scan_data_files(struct rrdengine_instance *ctx)
 {
     int ret;
@@ -249,16 +270,22 @@ static int scan_data_files(struct rrdengine_instance *ctx)
     struct rrdengine_journalfile *journalfile;
 
     ret = uv_fs_scandir(NULL, &req, ctx->dbfiles_path, 0, NULL);
-    assert(ret >= 0);
-    assert(req.result >= 0);
+    if (ret < 0) {
+        assert(req.result < 0);
+        uv_fs_req_cleanup(&req);
+        error("uv_fs_scandir(%s): %s", ctx->dbfiles_path, uv_strerror(ret));
+        ++ctx->stats.fs_errors;
+        rrd_stat_atomic_add(&global_fs_errors, 1);
+        return ret;
+    }
     info("Found %d files in path %s", ret, ctx->dbfiles_path);
 
     datafiles = callocz(MIN(ret, MAX_DATAFILES), sizeof(*datafiles));
     for (matched_files = 0 ; UV_EOF != uv_fs_scandir_next(&req, &dent) && matched_files < MAX_DATAFILES ; ) {
-        info("Scanning file \"%s\"", dent.name);
+        info("Scanning file \"%s/%s\"", ctx->dbfiles_path, dent.name);
         ret = sscanf(dent.name, DATAFILE_PREFIX RRDENG_FILE_NUMBER_SCAN_TMPL DATAFILE_EXTENSION, &tier, &no);
         if (2 == ret) {
-            info("Matched file \"%s\"", dent.name);
+            info("Matched file \"%s/%s\"", ctx->dbfiles_path, dent.name);
             datafile = mallocz(sizeof(*datafile));
             datafile_init(datafile, ctx, tier, no);
             datafiles[matched_files++] = datafile;
@@ -266,70 +293,133 @@ static int scan_data_files(struct rrdengine_instance *ctx)
     }
     uv_fs_req_cleanup(&req);
 
+    if (0 == matched_files) {
+        freez(datafiles);
+        return 0;
+    }
     if (matched_files == MAX_DATAFILES) {
         error("Warning: hit maximum database engine file limit of %d files", MAX_DATAFILES);
     }
     qsort(datafiles, matched_files, sizeof(*datafiles), scan_data_files_cmp);
+    /* TODO: change this when tiering is implemented */
+    ctx->last_fileno = datafiles[matched_files - 1]->fileno;
+
     for (failed_to_load = 0, i = 0 ; i < matched_files ; ++i) {
         datafile = datafiles[i];
         ret = load_data_file(datafile);
         if (0 != ret) {
-            free(datafile);
+            freez(datafile);
             ++failed_to_load;
-            continue;
+            break;
         }
         journalfile = mallocz(sizeof(*journalfile));
         datafile->journalfile = journalfile;
         journalfile_init(journalfile, datafile);
         ret = load_journal_file(ctx, journalfile, datafile);
         if (0 != ret) {
-            free(datafile);
-            free(journalfile);
+            close_data_file(datafile);
+            freez(datafile);
+            freez(journalfile);
             ++failed_to_load;
-            continue;
+            break;
         }
         datafile_list_insert(ctx, datafile);
         ctx->disk_space += datafile->pos + journalfile->pos;
     }
+    freez(datafiles);
     if (failed_to_load) {
-        error("%u files failed to load.", failed_to_load);
+        error("%u datafiles failed to load.", failed_to_load);
+        finalize_data_files(ctx);
+        return UV_EIO;
     }
-    free(datafiles);
 
-    return matched_files - failed_to_load;
+    return matched_files;
 }
 
 /* Creates a datafile and a journalfile pair */
-void create_new_datafile_pair(struct rrdengine_instance *ctx, unsigned tier, unsigned fileno)
+int create_new_datafile_pair(struct rrdengine_instance *ctx, unsigned tier, unsigned fileno)
 {
     struct rrdengine_datafile *datafile;
     struct rrdengine_journalfile *journalfile;
     int ret;
+    char path[RRDENG_PATH_MAX];
 
-    info("Creating new data and journal files.");
+    info("Creating new data and journal files in path %s", ctx->dbfiles_path);
     datafile = mallocz(sizeof(*datafile));
     datafile_init(datafile, ctx, tier, fileno);
     ret = create_data_file(datafile);
-    assert(!ret);
+    if (!ret) {
+        generate_datafilepath(datafile, path, sizeof(path));
+        info("Created data file \"%s\".", path);
+    } else {
+        goto error_after_datafile;
+    }
 
     journalfile = mallocz(sizeof(*journalfile));
     datafile->journalfile = journalfile;
     journalfile_init(journalfile, datafile);
     ret = create_journal_file(journalfile, datafile);
-    assert(!ret);
+    if (!ret) {
+        generate_journalfilepath(datafile, path, sizeof(path));
+        info("Created journal file \"%s\".", path);
+    } else {
+        goto error_after_journalfile;
+    }
     datafile_list_insert(ctx, datafile);
     ctx->disk_space += datafile->pos + journalfile->pos;
+
+    return 0;
+
+error_after_journalfile:
+    destroy_data_file(datafile);
+    freez(journalfile);
+error_after_datafile:
+    freez(datafile);
+    return ret;
 }
 
-/* Page cache must already be initialized. */
+/* Page cache must already be initialized.
+ * Return 0 on success.
+ */
 int init_data_files(struct rrdengine_instance *ctx)
 {
     int ret;
 
     ret = scan_data_files(ctx);
-    if (0 == ret) {
-        info("Data files not found, creating.");
-        create_new_datafile_pair(ctx, 1, 1);
+    if (ret < 0) {
+        error("Failed to scan path \"%s\".", ctx->dbfiles_path);
+        return ret;
+    } else if (0 == ret) {
+        info("Data files not found, creating in path \"%s\".", ctx->dbfiles_path);
+        ret = create_new_datafile_pair(ctx, 1, 1);
+        if (ret) {
+            error("Failed to create data and journal files in path \"%s\".", ctx->dbfiles_path);
+            return ret;
+        }
+        ctx->last_fileno = 1;
     }
+
     return 0;
+}
+
+void finalize_data_files(struct rrdengine_instance *ctx)
+{
+    struct rrdengine_datafile *datafile, *next_datafile;
+    struct rrdengine_journalfile *journalfile;
+    struct extent_info *extent, *next_extent;
+
+    for (datafile = ctx->datafiles.first ; datafile != NULL ; datafile = next_datafile) {
+        journalfile = datafile->journalfile;
+        next_datafile = datafile->next;
+
+        for (extent = datafile->extents.first ; extent != NULL ; extent = next_extent) {
+            next_extent = extent->next;
+            freez(extent);
+        }
+        close_journal_file(journalfile, datafile);
+        close_data_file(datafile);
+        freez(journalfile);
+        freez(datafile);
+
+    }
 }
\ No newline at end of file
diff --git a/database/engine/datafile.h b/database/engine/datafile.h
index c5c8f31f3..eeb11310b 100644
--- a/database/engine/datafile.h
+++ b/database/engine/datafile.h
@@ -26,7 +26,7 @@ struct extent_info {
     uint8_t number_of_pages;
     struct rrdengine_datafile *datafile;
     struct extent_info *next;
-    struct rrdeng_page_cache_descr *pages[];
+    struct rrdeng_page_descr *pages[];
 };
 
 struct rrdengine_df_extents {
@@ -55,9 +55,12 @@ struct rrdengine_datafile_list {
 extern void df_extent_insert(struct extent_info *extent);
 extern void datafile_list_insert(struct rrdengine_instance *ctx, struct rrdengine_datafile *datafile);
 extern void datafile_list_delete(struct rrdengine_instance *ctx, struct rrdengine_datafile *datafile);
+extern void generate_datafilepath(struct rrdengine_datafile *datafile, char *str, size_t maxlen);
+extern int close_data_file(struct rrdengine_datafile *datafile);
 extern int destroy_data_file(struct rrdengine_datafile *datafile);
 extern int create_data_file(struct rrdengine_datafile *datafile);
-extern void create_new_datafile_pair(struct rrdengine_instance *ctx, unsigned tier, unsigned fileno);
+extern int create_new_datafile_pair(struct rrdengine_instance *ctx, unsigned tier, unsigned fileno);
 extern int init_data_files(struct rrdengine_instance *ctx);
+extern void finalize_data_files(struct rrdengine_instance *ctx);
 
 #endif /* NETDATA_DATAFILE_H */
\ No newline at end of file
diff --git a/database/engine/journalfile.c b/database/engine/journalfile.c
index 44d8461db..30eaa0ec6 100644
--- a/database/engine/journalfile.c
+++ b/database/engine/journalfile.c
@@ -13,7 +13,7 @@ static void flush_transaction_buffer_cb(uv_fs_t* req)
 
     uv_fs_req_cleanup(req);
     free(io_descr->buf);
-    free(io_descr);
+    freez(io_descr);
 }
 
 /* Careful to always call this before creating a new journal file */
@@ -87,7 +87,7 @@ void * wal_get_transaction_buffer(struct rrdengine_worker_config* wc, unsigned s
     return ctx->commit_log.buf + buf_pos;
 }
 
-static void generate_journalfilepath(struct rrdengine_datafile *datafile, char *str, size_t maxlen)
+void generate_journalfilepath(struct rrdengine_datafile *datafile, char *str, size_t maxlen)
 {
     (void) snprintf(str, maxlen, "%s/" WALFILE_PREFIX RRDENG_FILE_NUMBER_PRINT_TMPL WALFILE_EXTENSION,
                     datafile->ctx->dbfiles_path, datafile->tier, datafile->fileno);
@@ -100,39 +100,62 @@ void journalfile_init(struct rrdengine_journalfile *journalfile, struct rrdengin
     journalfile->datafile = datafile;
 }
 
+int close_journal_file(struct rrdengine_journalfile *journalfile, struct rrdengine_datafile *datafile)
+{
+    struct rrdengine_instance *ctx = datafile->ctx;
+    uv_fs_t req;
+    int ret;
+    char path[RRDENG_PATH_MAX];
+
+    generate_journalfilepath(datafile, path, sizeof(path));
+
+    ret = uv_fs_close(NULL, &req, journalfile->file, NULL);
+    if (ret < 0) {
+        error("uv_fs_close(%s): %s", path, uv_strerror(ret));
+        ++ctx->stats.fs_errors;
+        rrd_stat_atomic_add(&global_fs_errors, 1);
+    }
+    uv_fs_req_cleanup(&req);
+
+    return ret;
+}
+
 int destroy_journal_file(struct rrdengine_journalfile *journalfile, struct rrdengine_datafile *datafile)
 {
     struct rrdengine_instance *ctx = datafile->ctx;
     uv_fs_t req;
-    int ret, fd;
-    char path[1024];
+    int ret;
+    char path[RRDENG_PATH_MAX];
+
+    generate_journalfilepath(datafile, path, sizeof(path));
 
     ret = uv_fs_ftruncate(NULL, &req, journalfile->file, 0, NULL);
     if (ret < 0) {
-        fatal("uv_fs_ftruncate: %s", uv_strerror(ret));
+        error("uv_fs_ftruncate(%s): %s", path, uv_strerror(ret));
+        ++ctx->stats.fs_errors;
+        rrd_stat_atomic_add(&global_fs_errors, 1);
     }
-    assert(0 == req.result);
     uv_fs_req_cleanup(&req);
 
     ret = uv_fs_close(NULL, &req, journalfile->file, NULL);
     if (ret < 0) {
-        fatal("uv_fs_close: %s", uv_strerror(ret));
-        exit(ret);
+        error("uv_fs_close(%s): %s", path, uv_strerror(ret));
+        ++ctx->stats.fs_errors;
+        rrd_stat_atomic_add(&global_fs_errors, 1);
     }
-    assert(0 == req.result);
     uv_fs_req_cleanup(&req);
 
-    generate_journalfilepath(datafile, path, sizeof(path));
-    fd = uv_fs_unlink(NULL, &req, path, NULL);
-    if (fd < 0) {
-        fatal("uv_fs_fsunlink: %s", uv_strerror(fd));
+    ret = uv_fs_unlink(NULL, &req, path, NULL);
+    if (ret < 0) {
+        error("uv_fs_fsunlink(%s): %s", path, uv_strerror(ret));
+        ++ctx->stats.fs_errors;
+        rrd_stat_atomic_add(&global_fs_errors, 1);
     }
-    assert(0 == req.result);
     uv_fs_req_cleanup(&req);
 
     ++ctx->stats.journalfile_deletions;
 
-    return 0;
+    return ret;
 }
 
 int create_journal_file(struct rrdengine_journalfile *journalfile, struct rrdengine_datafile *datafile)
@@ -143,21 +166,17 @@ int create_journal_file(struct rrdengine_journalfile *journalfile, struct rrdeng
     int ret, fd;
     struct rrdeng_jf_sb *superblock;
     uv_buf_t iov;
-    char path[1024];
+    char path[RRDENG_PATH_MAX];
 
     generate_journalfilepath(datafile, path, sizeof(path));
-    fd = uv_fs_open(NULL, &req, path, O_DIRECT | O_CREAT | O_RDWR | O_TRUNC,
-                    S_IRUSR | S_IWUSR, NULL);
+    fd = open_file_direct_io(path, O_CREAT | O_RDWR | O_TRUNC, &file);
     if (fd < 0) {
-        fatal("uv_fs_fsopen: %s", uv_strerror(fd));
+        ++ctx->stats.fs_errors;
+        rrd_stat_atomic_add(&global_fs_errors, 1);
+        return fd;
     }
-    assert(req.result >= 0);
-    file = req.result;
-    uv_fs_req_cleanup(&req);
-#ifdef __APPLE__
-    info("Disabling OS X caching for file \"%s\".", path);
-    fcntl(fd, F_NOCACHE, 1);
-#endif
+    journalfile->file = file;
+    ++ctx->stats.journalfile_creations;
 
     ret = posix_memalign((void *)&superblock, RRDFILE_ALIGNMENT, sizeof(*superblock));
     if (unlikely(ret)) {
@@ -170,19 +189,21 @@ int create_journal_file(struct rrdengine_journalfile *journalfile, struct rrdeng
 
     ret = uv_fs_write(NULL, &req, file, &iov, 1, 0, NULL);
     if (ret < 0) {
-        fatal("uv_fs_write: %s", uv_strerror(ret));
-    }
-    if (req.result < 0) {
-        fatal("uv_fs_write: %s", uv_strerror((int)req.result));
+        assert(req.result < 0);
+        error("uv_fs_write: %s", uv_strerror(ret));
+        ++ctx->stats.io_errors;
+        rrd_stat_atomic_add(&global_io_errors, 1);
     }
     uv_fs_req_cleanup(&req);
     free(superblock);
+    if (ret < 0) {
+        destroy_journal_file(journalfile, datafile);
+        return ret;
+    }
 
-    journalfile->file = file;
     journalfile->pos = sizeof(*superblock);
     ctx->stats.io_write_bytes += sizeof(*superblock);
     ++ctx->stats.io_write_requests;
-    ++ctx->stats.journalfile_creations;
 
     return 0;
 }
@@ -226,7 +247,7 @@ static void restore_extent_metadata(struct rrdengine_instance *ctx, struct rrden
 {
     struct page_cache *pg_cache = &ctx->pg_cache;
     unsigned i, count, payload_length, descr_size, valid_pages;
-    struct rrdeng_page_cache_descr *descr;
+    struct rrdeng_page_descr *descr;
     struct extent_info *extent;
     /* persistent structures */
     struct rrdeng_jf_store_data *jf_metric_data;
@@ -271,6 +292,8 @@ static void restore_extent_metadata(struct rrdengine_instance *ctx, struct rrden
             PValue = JudyHSIns(&pg_cache->metrics_index.JudyHS_array, temp_id, sizeof(uuid_t), PJE0);
             assert(NULL == *PValue); /* TODO: figure out concurrency model */
             *PValue = page_index = create_page_index(temp_id);
+            page_index->prev = pg_cache->metrics_index.last_page_index;
+            pg_cache->metrics_index.last_page_index = page_index;
             uv_rwlock_wrunlock(&pg_cache->metrics_index.lock);
         }
 
@@ -406,25 +429,17 @@ int load_journal_file(struct rrdengine_instance *ctx, struct rrdengine_journalfi
 {
     uv_fs_t req;
     uv_file file;
-    int ret, fd;
+    int ret, fd, error;
     uint64_t file_size, max_id;
-    char path[1024];
+    char path[RRDENG_PATH_MAX];
 
     generate_journalfilepath(datafile, path, sizeof(path));
-    fd = uv_fs_open(NULL, &req, path, O_DIRECT | O_RDWR, S_IRUSR | S_IWUSR, NULL);
+    fd = open_file_direct_io(path, O_RDWR, &file);
     if (fd < 0) {
-        /* if (UV_ENOENT != fd) */
-        error("uv_fs_fsopen: %s", uv_strerror(fd));
-        uv_fs_req_cleanup(&req);
+        ++ctx->stats.fs_errors;
+        rrd_stat_atomic_add(&global_fs_errors, 1);
         return fd;
     }
-    assert(req.result >= 0);
-    file = req.result;
-    uv_fs_req_cleanup(&req);
-#ifdef __APPLE__
-    info("Disabling OS X caching for file \"%s\".", path);
-    fcntl(fd, F_NOCACHE, 1);
-#endif
     info("Loading journal file \"%s\".", path);
 
     ret = check_file_properties(file, &file_size, sizeof(struct rrdeng_df_sb));
@@ -449,9 +464,15 @@ int load_journal_file(struct rrdengine_instance *ctx, struct rrdengine_journalfi
     return 0;
 
     error:
-    (void) uv_fs_close(NULL, &req, file, NULL);
+    error = ret;
+    ret = uv_fs_close(NULL, &req, file, NULL);
+    if (ret < 0) {
+        error("uv_fs_close(%s): %s", path, uv_strerror(ret));
+        ++ctx->stats.fs_errors;
+        rrd_stat_atomic_add(&global_fs_errors, 1);
+    }
     uv_fs_req_cleanup(&req);
-    return ret;
+    return error;
 }
 
 void init_commit_log(struct rrdengine_instance *ctx)
diff --git a/database/engine/journalfile.h b/database/engine/journalfile.h
index 50489aeee..0df66304d 100644
--- a/database/engine/journalfile.h
+++ b/database/engine/journalfile.h
@@ -33,9 +33,11 @@ struct transaction_commit_log {
     unsigned buf_size;
 };
 
+extern void generate_journalfilepath(struct rrdengine_datafile *datafile, char *str, size_t maxlen);
 extern void journalfile_init(struct rrdengine_journalfile *journalfile, struct rrdengine_datafile *datafile);
 extern void *wal_get_transaction_buffer(struct rrdengine_worker_config* wc, unsigned size);
 extern void wal_flush_transaction_buffer(struct rrdengine_worker_config* wc);
+extern int close_journal_file(struct rrdengine_journalfile *journalfile, struct rrdengine_datafile *datafile);
 extern int destroy_journal_file(struct rrdengine_journalfile *journalfile, struct rrdengine_datafile *datafile);
 extern int create_journal_file(struct rrdengine_journalfile *journalfile, struct rrdengine_datafile *datafile);
 extern int load_journal_file(struct rrdengine_instance *ctx, struct rrdengine_journalfile *journalfile,
diff --git a/database/engine/pagecache.c b/database/engine/pagecache.c
index c90947a67..124f2448b 100644
--- a/database/engine/pagecache.c
+++ b/database/engine/pagecache.c
@@ -8,28 +8,29 @@ static int pg_cache_try_evict_one_page_unsafe(struct rrdengine_instance *ctx);
 
 /* always inserts into tail */
 static inline void pg_cache_replaceQ_insert_unsafe(struct rrdengine_instance *ctx,
-                                                   struct rrdeng_page_cache_descr *descr)
+                                                   struct rrdeng_page_descr *descr)
 {
     struct page_cache *pg_cache = &ctx->pg_cache;
+    struct page_cache_descr *pg_cache_descr = descr->pg_cache_descr;
 
     if (likely(NULL != pg_cache->replaceQ.tail)) {
-        descr->prev = pg_cache->replaceQ.tail;
-        pg_cache->replaceQ.tail->next = descr;
+        pg_cache_descr->prev = pg_cache->replaceQ.tail;
+        pg_cache->replaceQ.tail->next = pg_cache_descr;
     }
     if (unlikely(NULL == pg_cache->replaceQ.head)) {
-        pg_cache->replaceQ.head = descr;
+        pg_cache->replaceQ.head = pg_cache_descr;
     }
-    pg_cache->replaceQ.tail = descr;
+    pg_cache->replaceQ.tail = pg_cache_descr;
 }
 
 static inline void pg_cache_replaceQ_delete_unsafe(struct rrdengine_instance *ctx,
-                                                   struct rrdeng_page_cache_descr *descr)
+                                                   struct rrdeng_page_descr *descr)
 {
     struct page_cache *pg_cache = &ctx->pg_cache;
-    struct rrdeng_page_cache_descr *prev, *next;
+    struct page_cache_descr *pg_cache_descr = descr->pg_cache_descr, *prev, *next;
 
-    prev = descr->prev;
-    next = descr->next;
+    prev = pg_cache_descr->prev;
+    next = pg_cache_descr->next;
 
     if (likely(NULL != prev)) {
         prev->next = next;
@@ -37,17 +38,17 @@ static inline void pg_cache_replaceQ_delete_unsafe(struct rrdengine_instance *ct
     if (likely(NULL != next)) {
         next->prev = prev;
     }
-    if (unlikely(descr == pg_cache->replaceQ.head)) {
+    if (unlikely(pg_cache_descr == pg_cache->replaceQ.head)) {
         pg_cache->replaceQ.head = next;
     }
-    if (unlikely(descr == pg_cache->replaceQ.tail)) {
+    if (unlikely(pg_cache_descr == pg_cache->replaceQ.tail)) {
         pg_cache->replaceQ.tail = prev;
     }
-    descr->prev = descr->next = NULL;
+    pg_cache_descr->prev = pg_cache_descr->next = NULL;
 }
 
 void pg_cache_replaceQ_insert(struct rrdengine_instance *ctx,
-                              struct rrdeng_page_cache_descr *descr)
+                              struct rrdeng_page_descr *descr)
 {
     struct page_cache *pg_cache = &ctx->pg_cache;
 
@@ -57,7 +58,7 @@ void pg_cache_replaceQ_insert(struct rrdengine_instance *ctx,
 }
 
 void pg_cache_replaceQ_delete(struct rrdengine_instance *ctx,
-                              struct rrdeng_page_cache_descr *descr)
+                              struct rrdeng_page_descr *descr)
 {
     struct page_cache *pg_cache = &ctx->pg_cache;
 
@@ -66,7 +67,7 @@ void pg_cache_replaceQ_delete(struct rrdengine_instance *ctx,
     uv_rwlock_wrunlock(&pg_cache->replaceQ.lock);
 }
 void pg_cache_replaceQ_set_hot(struct rrdengine_instance *ctx,
-                               struct rrdeng_page_cache_descr *descr)
+                               struct rrdeng_page_descr *descr)
 {
     struct page_cache *pg_cache = &ctx->pg_cache;
 
@@ -76,40 +77,28 @@ void pg_cache_replaceQ_set_hot(struct rrdengine_instance *ctx,
     uv_rwlock_wrunlock(&pg_cache->replaceQ.lock);
 }
 
-struct rrdeng_page_cache_descr *pg_cache_create_descr(void)
+struct rrdeng_page_descr *pg_cache_create_descr(void)
 {
-    struct rrdeng_page_cache_descr *descr;
+    struct rrdeng_page_descr *descr;
 
     descr = mallocz(sizeof(*descr));
-    descr->page = NULL;
     descr->page_length = 0;
     descr->start_time = INVALID_TIME;
     descr->end_time = INVALID_TIME;
     descr->id = NULL;
     descr->extent = NULL;
-    descr->flags = 0;
-    descr->prev = descr->next = descr->private = NULL;
-    descr->refcnt = 0;
-    descr->waiters = 0;
-    descr->handle = NULL;
-    assert(0 == uv_cond_init(&descr->cond));
-    assert(0 == uv_mutex_init(&descr->mutex));
+    descr->pg_cache_descr_state = 0;
+    descr->pg_cache_descr = NULL;
 
     return descr;
 }
 
-void pg_cache_destroy_descr(struct rrdeng_page_cache_descr *descr)
-{
-    uv_cond_destroy(&descr->cond);
-    uv_mutex_destroy(&descr->mutex);
-    free(descr);
-}
-
 /* The caller must hold page descriptor lock. */
-void pg_cache_wake_up_waiters_unsafe(struct rrdeng_page_cache_descr *descr)
+void pg_cache_wake_up_waiters_unsafe(struct rrdeng_page_descr *descr)
 {
-    if (descr->waiters)
-        uv_cond_broadcast(&descr->cond);
+    struct page_cache_descr *pg_cache_descr = descr->pg_cache_descr;
+    if (pg_cache_descr->waiters)
+        uv_cond_broadcast(&pg_cache_descr->cond);
 }
 
 /*
@@ -117,11 +106,13 @@ void pg_cache_wake_up_waiters_unsafe(struct rrdeng_page_cache_descr *descr)
  * The lock will be released and re-acquired. The descriptor is not guaranteed
  * to exist after this function returns.
  */
-void pg_cache_wait_event_unsafe(struct rrdeng_page_cache_descr *descr)
+void pg_cache_wait_event_unsafe(struct rrdeng_page_descr *descr)
 {
-    ++descr->waiters;
-    uv_cond_wait(&descr->cond, &descr->mutex);
-    --descr->waiters;
+    struct page_cache_descr *pg_cache_descr = descr->pg_cache_descr;
+
+    ++pg_cache_descr->waiters;
+    uv_cond_wait(&pg_cache_descr->cond, &pg_cache_descr->mutex);
+    --pg_cache_descr->waiters;
 }
 
 /*
@@ -129,14 +120,15 @@ void pg_cache_wait_event_unsafe(struct rrdeng_page_cache_descr *descr)
  * The lock will be released and re-acquired. The descriptor is not guaranteed
  * to exist after this function returns.
  */
-unsigned long pg_cache_wait_event(struct rrdeng_page_cache_descr *descr)
+unsigned long pg_cache_wait_event(struct rrdengine_instance *ctx, struct rrdeng_page_descr *descr)
 {
+    struct page_cache_descr *pg_cache_descr = descr->pg_cache_descr;
     unsigned long flags;
 
-    uv_mutex_lock(&descr->mutex);
+    rrdeng_page_descr_mutex_lock(ctx, descr);
     pg_cache_wait_event_unsafe(descr);
-    flags = descr->flags;
-    uv_mutex_unlock(&descr->mutex);
+    flags = pg_cache_descr->flags;
+    rrdeng_page_descr_mutex_unlock(ctx, descr);
 
     return flags;
 }
@@ -146,15 +138,17 @@ unsigned long pg_cache_wait_event(struct rrdeng_page_cache_descr *descr)
  * Gets a reference to the page descriptor.
  * Returns 1 on success and 0 on failure.
  */
-int pg_cache_try_get_unsafe(struct rrdeng_page_cache_descr *descr, int exclusive_access)
+int pg_cache_try_get_unsafe(struct rrdeng_page_descr *descr, int exclusive_access)
 {
-    if ((descr->flags & (RRD_PAGE_LOCKED | RRD_PAGE_READ_PENDING)) ||
-        (exclusive_access && descr->refcnt)) {
+    struct page_cache_descr *pg_cache_descr = descr->pg_cache_descr;
+
+    if ((pg_cache_descr->flags & (RRD_PAGE_LOCKED | RRD_PAGE_READ_PENDING)) ||
+        (exclusive_access && pg_cache_descr->refcnt)) {
         return 0;
     }
     if (exclusive_access)
-        descr->flags |= RRD_PAGE_LOCKED;
-    ++descr->refcnt;
+        pg_cache_descr->flags |= RRD_PAGE_LOCKED;
+    ++pg_cache_descr->refcnt;
 
     return 1;
 }
@@ -163,10 +157,12 @@ int pg_cache_try_get_unsafe(struct rrdeng_page_cache_descr *descr, int exclusive
  * The caller must hold page descriptor lock.
  * Same return values as pg_cache_try_get_unsafe() without doing anything.
  */
-int pg_cache_can_get_unsafe(struct rrdeng_page_cache_descr *descr, int exclusive_access)
+int pg_cache_can_get_unsafe(struct rrdeng_page_descr *descr, int exclusive_access)
 {
-    if ((descr->flags & (RRD_PAGE_LOCKED | RRD_PAGE_READ_PENDING)) ||
-        (exclusive_access && descr->refcnt)) {
+    struct page_cache_descr *pg_cache_descr = descr->pg_cache_descr;
+
+    if ((pg_cache_descr->flags & (RRD_PAGE_LOCKED | RRD_PAGE_READ_PENDING)) ||
+        (exclusive_access && pg_cache_descr->refcnt)) {
         return 0;
     }
 
@@ -177,23 +173,24 @@ int pg_cache_can_get_unsafe(struct rrdeng_page_cache_descr *descr, int exclusive
  * The caller must hold the page descriptor lock.
  * This function may block doing cleanup.
  */
-void pg_cache_put_unsafe(struct rrdeng_page_cache_descr *descr)
+void pg_cache_put_unsafe(struct rrdeng_page_descr *descr)
 {
-    descr->flags &= ~RRD_PAGE_LOCKED;
-    if (0 == --descr->refcnt) {
+    struct page_cache_descr *pg_cache_descr = descr->pg_cache_descr;
+
+    pg_cache_descr->flags &= ~RRD_PAGE_LOCKED;
+    if (0 == --pg_cache_descr->refcnt) {
         pg_cache_wake_up_waiters_unsafe(descr);
     }
-    /* TODO: perform cleanup */
 }
 
 /*
  * This function may block doing cleanup.
  */
-void pg_cache_put(struct rrdeng_page_cache_descr *descr)
+void pg_cache_put(struct rrdengine_instance *ctx, struct rrdeng_page_descr *descr)
 {
-    uv_mutex_lock(&descr->mutex);
+    rrdeng_page_descr_mutex_lock(ctx, descr);
     pg_cache_put_unsafe(descr);
-    uv_mutex_unlock(&descr->mutex);
+    rrdeng_page_descr_mutex_unlock(ctx, descr);
 }
 
 /* The caller must hold the page cache lock */
@@ -224,7 +221,7 @@ static void pg_cache_reserve_pages(struct rrdengine_instance *ctx, unsigned numb
 
     uv_rwlock_wrlock(&pg_cache->pg_cache_rwlock);
     if (pg_cache->populated_pages + number >= ctx->max_cache_pages + 1)
-        debug(D_RRDENGINE, "=================================\nPage cache full. Reserving %u pages.\n=================================",
+        debug(D_RRDENGINE, "==Page cache full. Reserving %u pages.==",
                 number);
     while (pg_cache->populated_pages + number >= ctx->max_cache_pages + 1) {
         if (!pg_cache_try_evict_one_page_unsafe(ctx)) {
@@ -266,7 +263,7 @@ static int pg_cache_try_reserve_pages(struct rrdengine_instance *ctx, unsigned n
     uv_rwlock_wrlock(&pg_cache->pg_cache_rwlock);
     if (pg_cache->populated_pages + number >= ctx->cache_pages_low_watermark + 1) {
         debug(D_RRDENGINE,
-              "=================================\nPage cache full. Trying to reserve %u pages.\n=================================",
+              "==Page cache full. Trying to reserve %u pages.==",
               number);
         do {
             if (!pg_cache_try_evict_one_page_unsafe(ctx))
@@ -286,18 +283,20 @@ static int pg_cache_try_reserve_pages(struct rrdengine_instance *ctx, unsigned n
 }
 
 /* The caller must hold the page cache and the page descriptor locks in that order */
-static void pg_cache_evict_unsafe(struct rrdengine_instance *ctx, struct rrdeng_page_cache_descr *descr)
+static void pg_cache_evict_unsafe(struct rrdengine_instance *ctx, struct rrdeng_page_descr *descr)
 {
-    free(descr->page);
-    descr->page = NULL;
-    descr->flags &= ~RRD_PAGE_POPULATED;
+    struct page_cache_descr *pg_cache_descr = descr->pg_cache_descr;
+
+    freez(pg_cache_descr->page);
+    pg_cache_descr->page = NULL;
+    pg_cache_descr->flags &= ~RRD_PAGE_POPULATED;
     pg_cache_release_pages_unsafe(ctx, 1);
     ++ctx->stats.pg_cache_evictions;
 }
 
 /*
  * The caller must hold the page cache lock.
- * Lock order: page cache -> replaceQ -> descriptor
+ * Lock order: page cache -> replaceQ -> page descriptor
  * This function iterates all pages and tries to evict one.
  * If it fails it sets in_flight_descr to the oldest descriptor that has write-back in progress,
  * or it sets it to NULL if no write-back is in progress.
@@ -308,36 +307,40 @@ static int pg_cache_try_evict_one_page_unsafe(struct rrdengine_instance *ctx)
 {
     struct page_cache *pg_cache = &ctx->pg_cache;
     unsigned long old_flags;
-    struct rrdeng_page_cache_descr *descr;
+    struct rrdeng_page_descr *descr;
+    struct page_cache_descr *pg_cache_descr = NULL;
 
     uv_rwlock_wrlock(&pg_cache->replaceQ.lock);
-    for (descr = pg_cache->replaceQ.head ; NULL != descr ; descr = descr->next) {
-        uv_mutex_lock(&descr->mutex);
-        old_flags = descr->flags;
+    for (pg_cache_descr = pg_cache->replaceQ.head ; NULL != pg_cache_descr ; pg_cache_descr = pg_cache_descr->next) {
+        descr = pg_cache_descr->descr;
+
+        rrdeng_page_descr_mutex_lock(ctx, descr);
+        old_flags = pg_cache_descr->flags;
         if ((old_flags & RRD_PAGE_POPULATED) && !(old_flags & RRD_PAGE_DIRTY) && pg_cache_try_get_unsafe(descr, 1)) {
             /* must evict */
             pg_cache_evict_unsafe(ctx, descr);
             pg_cache_put_unsafe(descr);
-            uv_mutex_unlock(&descr->mutex);
             pg_cache_replaceQ_delete_unsafe(ctx, descr);
+
+            rrdeng_page_descr_mutex_unlock(ctx, descr);
             uv_rwlock_wrunlock(&pg_cache->replaceQ.lock);
 
+            rrdeng_try_deallocate_pg_cache_descr(ctx, descr);
+
             return 1;
         }
-        uv_mutex_unlock(&descr->mutex);
-    };
+        rrdeng_page_descr_mutex_unlock(ctx, descr);
+    }
     uv_rwlock_wrunlock(&pg_cache->replaceQ.lock);
 
     /* failed to evict */
     return 0;
 }
 
-/*
- * TODO: last waiter frees descriptor ?
- */
-void pg_cache_punch_hole(struct rrdengine_instance *ctx, struct rrdeng_page_cache_descr *descr)
+void pg_cache_punch_hole(struct rrdengine_instance *ctx, struct rrdeng_page_descr *descr, uint8_t remove_dirty)
 {
     struct page_cache *pg_cache = &ctx->pg_cache;
+    struct page_cache_descr *pg_cache_descr = NULL;
     Pvoid_t *PValue;
     struct pg_cache_page_index *page_index;
     int ret;
@@ -353,8 +356,9 @@ void pg_cache_punch_hole(struct rrdengine_instance *ctx, struct rrdeng_page_cach
     uv_rwlock_wrunlock(&page_index->lock);
     if (unlikely(0 == ret)) {
         error("Page under deletion was not in index.");
-        if (unlikely(debug_flags & D_RRDENGINE))
-            print_page_cache_descr(descr);
+        if (unlikely(debug_flags & D_RRDENGINE)) {
+            print_page_descr(descr);
+        }
         goto destroy;
     }
     assert(1 == ret);
@@ -364,23 +368,26 @@ void pg_cache_punch_hole(struct rrdengine_instance *ctx, struct rrdeng_page_cach
     --pg_cache->page_descriptors;
     uv_rwlock_wrunlock(&pg_cache->pg_cache_rwlock);
 
-    uv_mutex_lock(&descr->mutex);
+    rrdeng_page_descr_mutex_lock(ctx, descr);
+    pg_cache_descr = descr->pg_cache_descr;
     while (!pg_cache_try_get_unsafe(descr, 1)) {
         debug(D_RRDENGINE, "%s: Waiting for locked page:", __func__);
-        if(unlikely(debug_flags & D_RRDENGINE))
-            print_page_cache_descr(descr);
-        pg_cache_wait_event_unsafe(descr);
-    }
-    /* even a locked page could be dirty */
-    while (unlikely(descr->flags & RRD_PAGE_DIRTY)) {
-        debug(D_RRDENGINE, "%s: Found dirty page, waiting for it to be flushed:", __func__);
         if (unlikely(debug_flags & D_RRDENGINE))
             print_page_cache_descr(descr);
         pg_cache_wait_event_unsafe(descr);
     }
-    uv_mutex_unlock(&descr->mutex);
+    if (!remove_dirty) {
+        /* even a locked page could be dirty */
+        while (unlikely(pg_cache_descr->flags & RRD_PAGE_DIRTY)) {
+            debug(D_RRDENGINE, "%s: Found dirty page, waiting for it to be flushed:", __func__);
+            if (unlikely(debug_flags & D_RRDENGINE))
+                print_page_cache_descr(descr);
+            pg_cache_wait_event_unsafe(descr);
+        }
+    }
+    rrdeng_page_descr_mutex_unlock(ctx, descr);
 
-    if (descr->flags & RRD_PAGE_POPULATED) {
+    if (pg_cache_descr->flags & RRD_PAGE_POPULATED) {
         /* only after locking can it be safely deleted from LRU */
         pg_cache_replaceQ_delete(ctx, descr);
 
@@ -388,13 +395,15 @@ void pg_cache_punch_hole(struct rrdengine_instance *ctx, struct rrdeng_page_cach
         pg_cache_evict_unsafe(ctx, descr);
         uv_rwlock_wrunlock(&pg_cache->pg_cache_rwlock);
     }
-    pg_cache_put(descr);
+    pg_cache_put(ctx, descr);
+
+    rrdeng_destroy_pg_cache_descr(ctx, pg_cache_descr);
 destroy:
-    pg_cache_destroy_descr(descr);
+    freez(descr);
     pg_cache_update_metric_times(page_index);
 }
 
-static inline int is_page_in_time_range(struct rrdeng_page_cache_descr *descr, usec_t start_time, usec_t end_time)
+static inline int is_page_in_time_range(struct rrdeng_page_descr *descr, usec_t start_time, usec_t end_time)
 {
     usec_t pg_start, pg_end;
 
@@ -405,13 +414,13 @@ static inline int is_page_in_time_range(struct rrdeng_page_cache_descr *descr, u
            (pg_start >= start_time && pg_start <= end_time);
 }
 
-static inline int is_point_in_time_in_page(struct rrdeng_page_cache_descr *descr, usec_t point_in_time)
+static inline int is_point_in_time_in_page(struct rrdeng_page_descr *descr, usec_t point_in_time)
 {
     return (point_in_time >= descr->start_time && point_in_time <= descr->end_time);
 }
 
 /* Update metric oldest and latest timestamps efficiently when adding new values */
-void pg_cache_add_new_metric_time(struct pg_cache_page_index *page_index, struct rrdeng_page_cache_descr *descr)
+void pg_cache_add_new_metric_time(struct pg_cache_page_index *page_index, struct rrdeng_page_descr *descr)
 {
     usec_t oldest_time = page_index->oldest_time;
     usec_t latest_time = page_index->latest_time;
@@ -429,7 +438,7 @@ void pg_cache_update_metric_times(struct pg_cache_page_index *page_index)
 {
     Pvoid_t *firstPValue, *lastPValue;
     Word_t firstIndex, lastIndex;
-    struct rrdeng_page_cache_descr *descr;
+    struct rrdeng_page_descr *descr;
     usec_t oldest_time = INVALID_TIME;
     usec_t latest_time = INVALID_TIME;
 
@@ -460,16 +469,23 @@ void pg_cache_update_metric_times(struct pg_cache_page_index *page_index)
 
 /* If index is NULL lookup by UUID (descr->id) */
 void pg_cache_insert(struct rrdengine_instance *ctx, struct pg_cache_page_index *index,
-                     struct rrdeng_page_cache_descr *descr)
+                     struct rrdeng_page_descr *descr)
 {
     struct page_cache *pg_cache = &ctx->pg_cache;
     Pvoid_t *PValue;
     struct pg_cache_page_index *page_index;
+    unsigned long pg_cache_descr_state = descr->pg_cache_descr_state;
+
+    if (0 != pg_cache_descr_state) {
+        /* there is page cache descriptor pre-allocated state */
+        struct page_cache_descr *pg_cache_descr = descr->pg_cache_descr;
 
-    if (descr->flags & RRD_PAGE_POPULATED) {
-        pg_cache_reserve_pages(ctx, 1);
-        if (!(descr->flags & RRD_PAGE_DIRTY))
-            pg_cache_replaceQ_insert(ctx, descr);
+        assert(pg_cache_descr_state & PG_CACHE_DESCR_ALLOCATED);
+        if (pg_cache_descr->flags & RRD_PAGE_POPULATED) {
+            pg_cache_reserve_pages(ctx, 1);
+            if (!(pg_cache_descr->flags & RRD_PAGE_DIRTY))
+                pg_cache_replaceQ_insert(ctx, descr);
+        }
     }
 
     if (unlikely(NULL == index)) {
@@ -503,7 +519,8 @@ struct pg_cache_page_index *
         pg_cache_preload(struct rrdengine_instance *ctx, uuid_t *id, usec_t start_time, usec_t end_time)
 {
     struct page_cache *pg_cache = &ctx->pg_cache;
-    struct rrdeng_page_cache_descr *descr = NULL, *preload_array[PAGE_CACHE_MAX_PRELOAD_PAGES];
+    struct rrdeng_page_descr *descr = NULL, *preload_array[PAGE_CACHE_MAX_PRELOAD_PAGES];
+    struct page_cache_descr *pg_cache_descr = NULL;
     int i, j, k, count, found;
     unsigned long flags;
     Pvoid_t *PValue;
@@ -557,12 +574,13 @@ struct pg_cache_page_index *
 
         if (unlikely(0 == descr->page_length))
             continue;
-        uv_mutex_lock(&descr->mutex);
-        flags = descr->flags;
+        rrdeng_page_descr_mutex_lock(ctx, descr);
+        pg_cache_descr = descr->pg_cache_descr;
+        flags = pg_cache_descr->flags;
         if (pg_cache_can_get_unsafe(descr, 0)) {
             if (flags & RRD_PAGE_POPULATED) {
                 /* success */
-                uv_mutex_unlock(&descr->mutex);
+                rrdeng_page_descr_mutex_unlock(ctx, descr);
                 debug(D_RRDENGINE, "%s: Page was found in memory.", __func__);
                 continue;
             }
@@ -570,19 +588,19 @@ struct pg_cache_page_index *
         if (!(flags & RRD_PAGE_POPULATED) && pg_cache_try_get_unsafe(descr, 1)) {
             preload_array[count++] = descr;
             if (PAGE_CACHE_MAX_PRELOAD_PAGES == count) {
-                uv_mutex_unlock(&descr->mutex);
+                rrdeng_page_descr_mutex_unlock(ctx, descr);
                 break;
             }
         }
-        uv_mutex_unlock(&descr->mutex);
+        rrdeng_page_descr_mutex_unlock(ctx, descr);
 
-    };
+    }
     uv_rwlock_rdunlock(&page_index->lock);
 
     failed_to_reserve = 0;
     for (i = 0 ; i < count && !failed_to_reserve ; ++i) {
         struct rrdeng_cmd cmd;
-        struct rrdeng_page_cache_descr *next;
+        struct rrdeng_page_descr *next;
 
         descr = preload_array[i];
         if (NULL == descr) {
@@ -622,7 +640,7 @@ struct pg_cache_page_index *
             if (NULL == descr) {
                 continue;
             }
-            pg_cache_put(descr);
+            pg_cache_put(ctx, descr);
         }
     }
     if (!count) {
@@ -637,12 +655,13 @@ struct pg_cache_page_index *
  * When point_in_time is INVALID_TIME get any page.
  * If index is NULL lookup by UUID (id).
  */
-struct rrdeng_page_cache_descr *
+struct rrdeng_page_descr *
         pg_cache_lookup(struct rrdengine_instance *ctx, struct pg_cache_page_index *index, uuid_t *id,
                         usec_t point_in_time)
 {
     struct page_cache *pg_cache = &ctx->pg_cache;
-    struct rrdeng_page_cache_descr *descr = NULL;
+    struct rrdeng_page_descr *descr = NULL;
+    struct page_cache_descr *pg_cache_descr = NULL;
     unsigned long flags;
     Pvoid_t *PValue;
     struct pg_cache_page_index *page_index;
@@ -682,11 +701,12 @@ struct rrdeng_page_cache_descr *
             pg_cache_release_pages(ctx, 1);
             return NULL;
         }
-        uv_mutex_lock(&descr->mutex);
-        flags = descr->flags;
+        rrdeng_page_descr_mutex_lock(ctx, descr);
+        pg_cache_descr = descr->pg_cache_descr;
+        flags = pg_cache_descr->flags;
         if ((flags & RRD_PAGE_POPULATED) && pg_cache_try_get_unsafe(descr, 0)) {
             /* success */
-            uv_mutex_unlock(&descr->mutex);
+            rrdeng_page_descr_mutex_unlock(ctx, descr);
             debug(D_RRDENGINE, "%s: Page was found in memory.", __func__);
             break;
         }
@@ -702,14 +722,14 @@ struct rrdeng_page_cache_descr *
             debug(D_RRDENGINE, "%s: Waiting for page to be asynchronously read from disk:", __func__);
             if(unlikely(debug_flags & D_RRDENGINE))
                 print_page_cache_descr(descr);
-            while (!(descr->flags & RRD_PAGE_POPULATED)) {
+            while (!(pg_cache_descr->flags & RRD_PAGE_POPULATED)) {
                 pg_cache_wait_event_unsafe(descr);
             }
             /* success */
             /* Downgrade exclusive reference to allow other readers */
-            descr->flags &= ~RRD_PAGE_LOCKED;
+            pg_cache_descr->flags &= ~RRD_PAGE_LOCKED;
             pg_cache_wake_up_waiters_unsafe(descr);
-            uv_mutex_unlock(&descr->mutex);
+            rrdeng_page_descr_mutex_unlock(ctx, descr);
             rrd_stat_atomic_add(&ctx->stats.pg_cache_misses, 1);
             return descr;
         }
@@ -720,7 +740,7 @@ struct rrdeng_page_cache_descr *
         if (!(flags & RRD_PAGE_POPULATED))
             page_not_in_cache = 1;
         pg_cache_wait_event_unsafe(descr);
-        uv_mutex_unlock(&descr->mutex);
+        rrdeng_page_descr_mutex_unlock(ctx, descr);
 
         /* reset scan to find again */
         uv_rwlock_rdlock(&page_index->lock);
@@ -747,6 +767,7 @@ struct pg_cache_page_index *create_page_index(uuid_t *id)
     assert(0 == uv_rwlock_init(&page_index->lock));
     page_index->oldest_time = INVALID_TIME;
     page_index->latest_time = INVALID_TIME;
+    page_index->prev = NULL;
 
     return page_index;
 }
@@ -756,6 +777,7 @@ static void init_metrics_index(struct rrdengine_instance *ctx)
     struct page_cache *pg_cache = &ctx->pg_cache;
 
     pg_cache->metrics_index.JudyHS_array = (Pvoid_t) NULL;
+    pg_cache->metrics_index.last_page_index = NULL;
     assert(0 == uv_rwlock_init(&pg_cache->metrics_index.lock));
 }
 
@@ -789,4 +811,65 @@ void init_page_cache(struct rrdengine_instance *ctx)
     init_metrics_index(ctx);
     init_replaceQ(ctx);
     init_commited_page_index(ctx);
+}
+
+void free_page_cache(struct rrdengine_instance *ctx)
+{
+    struct page_cache *pg_cache = &ctx->pg_cache;
+    Word_t ret_Judy, bytes_freed = 0;
+    Pvoid_t *PValue;
+    struct pg_cache_page_index *page_index, *prev_page_index;
+    Word_t Index;
+    struct rrdeng_page_descr *descr;
+    struct page_cache_descr *pg_cache_descr;
+
+    /* Free commited page index */
+    ret_Judy = JudyLFreeArray(&pg_cache->commited_page_index.JudyL_array, PJE0);
+    assert(NULL == pg_cache->commited_page_index.JudyL_array);
+    bytes_freed += ret_Judy;
+
+    for (page_index = pg_cache->metrics_index.last_page_index ;
+         page_index != NULL ;
+         page_index = prev_page_index) {
+        prev_page_index = page_index->prev;
+
+        /* Find first page in range */
+        Index = (Word_t) 0;
+        PValue = JudyLFirst(page_index->JudyL_array, &Index, PJE0);
+        if (likely(NULL != PValue)) {
+            descr = *PValue;
+        }
+        while (descr != NULL) {
+            /* Iterate all page descriptors of this metric */
+
+            if (descr->pg_cache_descr_state & PG_CACHE_DESCR_ALLOCATED) {
+                /* Check rrdenglocking.c */
+                pg_cache_descr = descr->pg_cache_descr;
+                if (pg_cache_descr->flags & RRD_PAGE_POPULATED) {
+                    freez(pg_cache_descr->page);
+                    bytes_freed += RRDENG_BLOCK_SIZE;
+                }
+                rrdeng_destroy_pg_cache_descr(ctx, pg_cache_descr);
+                bytes_freed += sizeof(*pg_cache_descr);
+            }
+            freez(descr);
+            bytes_freed += sizeof(*descr);
+
+            PValue = JudyLNext(page_index->JudyL_array, &Index, PJE0);
+            descr = unlikely(NULL == PValue) ? NULL : *PValue;
+        }
+
+        /* Free page index */
+        ret_Judy = JudyLFreeArray(&page_index->JudyL_array, PJE0);
+        assert(NULL == page_index->JudyL_array);
+        bytes_freed += ret_Judy;
+        freez(page_index);
+        bytes_freed += sizeof(*page_index);
+    }
+    /* Free metrics index */
+    ret_Judy = JudyHSFreeArray(&pg_cache->metrics_index.JudyHS_array, PJE0);
+    assert(NULL == pg_cache->metrics_index.JudyHS_array);
+    bytes_freed += ret_Judy;
+
+    info("Freed %lu bytes of memory from page cache.", bytes_freed);
 }
\ No newline at end of file
diff --git a/database/engine/pagecache.h b/database/engine/pagecache.h
index d1e29aaab..b5670f82a 100644
--- a/database/engine/pagecache.h
+++ b/database/engine/pagecache.h
@@ -5,9 +5,10 @@
 
 #include "rrdengine.h"
 
-/* Forward declerations */
+/* Forward declarations */
 struct rrdengine_instance;
 struct extent_info;
+struct rrdeng_page_descr;
 
 #define INVALID_TIME (0)
 
@@ -18,24 +19,46 @@ struct extent_info;
 #define RRD_PAGE_WRITE_PENDING  (1LU << 3)
 #define RRD_PAGE_POPULATED      (1LU << 4)
 
-struct rrdeng_page_cache_descr {
+struct page_cache_descr {
+    struct rrdeng_page_descr *descr; /* parent descriptor */
     void *page;
-    uint32_t page_length;
-    usec_t start_time;
-    usec_t end_time;
-    uuid_t *id; /* never changes */
-    struct extent_info *extent;
     unsigned long flags;
-    void *private;
-    struct rrdeng_page_cache_descr *prev;
-    struct rrdeng_page_cache_descr *next;
+    struct page_cache_descr *prev; /* LRU */
+    struct page_cache_descr *next; /* LRU */
 
-    /* TODO: move waiter logic to concurrency table */
     unsigned refcnt;
     uv_mutex_t mutex; /* always take it after the page cache lock or after the commit lock */
     uv_cond_t cond;
     unsigned waiters;
-    struct rrdeng_collect_handle *handle; /* API user */
+};
+
+/* Page cache descriptor flags, state = 0 means no descriptor */
+#define PG_CACHE_DESCR_ALLOCATED    (1LU << 0)
+#define PG_CACHE_DESCR_DESTROY      (1LU << 1)
+#define PG_CACHE_DESCR_LOCKED       (1LU << 2)
+#define PG_CACHE_DESCR_SHIFT        (3)
+#define PG_CACHE_DESCR_USERS_MASK   (((unsigned long)-1) << PG_CACHE_DESCR_SHIFT)
+#define PG_CACHE_DESCR_FLAGS_MASK   (((unsigned long)-1) >> (BITS_PER_ULONG - PG_CACHE_DESCR_SHIFT))
+
+/*
+ * Page cache descriptor state bits (works for both 32-bit and 64-bit architectures):
+ *
+ * 63    ...     31   ...     3 |          2 |          1 |          0|
+ * -----------------------------+------------+------------+-----------|
+ * number of descriptor users   |    DESTROY |     LOCKED | ALLOCATED |
+ */
+struct rrdeng_page_descr {
+    uint32_t page_length;
+    usec_t start_time;
+    usec_t end_time;
+    uuid_t *id; /* never changes */
+    struct extent_info *extent;
+
+    /* points to ephemeral page cache descriptor if the page resides in the cache */
+    struct page_cache_descr *pg_cache_descr;
+
+    /* Compare-And-Swap target for page cache descriptor allocation algorithm */
+    volatile unsigned long pg_cache_descr_state;
 };
 
 #define PAGE_CACHE_MAX_PRELOAD_PAGES    (256)
@@ -61,12 +84,15 @@ struct pg_cache_page_index {
      * It's also written by the data deletion workqueue when data collection is disabled for this metric.
      */
     usec_t latest_time;
+
+    struct pg_cache_page_index *prev;
 };
 
 /* maps UUIDs to page indices */
 struct pg_cache_metrics_index {
     uv_rwlock_t lock;
     Pvoid_t JudyHS_array;
+    struct pg_cache_page_index *last_page_index;
 };
 
 /* gathers dirty pages to be written on disk */
@@ -85,12 +111,15 @@ struct pg_cache_commited_page_index {
     unsigned nr_commited_pages;
 };
 
-/* gathers populated pages to be evicted */
+/*
+ * Gathers populated pages to be evicted.
+ * Relies on page cache descriptors being there as it uses their memory.
+ */
 struct pg_cache_replaceQ {
     uv_rwlock_t lock; /* LRU lock */
 
-    struct rrdeng_page_cache_descr *head; /* LRU */
-    struct rrdeng_page_cache_descr *tail; /* MRU */
+    struct page_cache_descr *head; /* LRU */
+    struct page_cache_descr *tail; /* MRU */
 };
 
 struct page_cache { /* TODO: add statistics */
@@ -104,29 +133,31 @@ struct page_cache { /* TODO: add statistics */
     unsigned populated_pages;
 };
 
-extern void pg_cache_wake_up_waiters_unsafe(struct rrdeng_page_cache_descr *descr);
-extern void pg_cache_wait_event_unsafe(struct rrdeng_page_cache_descr *descr);
-extern unsigned long pg_cache_wait_event(struct rrdeng_page_cache_descr *descr);
+extern void pg_cache_wake_up_waiters_unsafe(struct rrdeng_page_descr *descr);
+extern void pg_cache_wait_event_unsafe(struct rrdeng_page_descr *descr);
+extern unsigned long pg_cache_wait_event(struct rrdengine_instance *ctx, struct rrdeng_page_descr *descr);
 extern void pg_cache_replaceQ_insert(struct rrdengine_instance *ctx,
-                                     struct rrdeng_page_cache_descr *descr);
+                                     struct rrdeng_page_descr *descr);
 extern void pg_cache_replaceQ_delete(struct rrdengine_instance *ctx,
-                                     struct rrdeng_page_cache_descr *descr);
+                                     struct rrdeng_page_descr *descr);
 extern void pg_cache_replaceQ_set_hot(struct rrdengine_instance *ctx,
-                                      struct rrdeng_page_cache_descr *descr);
-extern struct rrdeng_page_cache_descr *pg_cache_create_descr(void);
-extern void pg_cache_put_unsafe(struct rrdeng_page_cache_descr *descr);
-extern void pg_cache_put(struct rrdeng_page_cache_descr *descr);
+                                      struct rrdeng_page_descr *descr);
+extern struct rrdeng_page_descr *pg_cache_create_descr(void);
+extern int pg_cache_try_get_unsafe(struct rrdeng_page_descr *descr, int exclusive_access);
+extern void pg_cache_put_unsafe(struct rrdeng_page_descr *descr);
+extern void pg_cache_put(struct rrdengine_instance *ctx, struct rrdeng_page_descr *descr);
 extern void pg_cache_insert(struct rrdengine_instance *ctx, struct pg_cache_page_index *index,
-                            struct rrdeng_page_cache_descr *descr);
-extern void pg_cache_punch_hole(struct rrdengine_instance *ctx, struct rrdeng_page_cache_descr *descr);
+                            struct rrdeng_page_descr *descr);
+extern void pg_cache_punch_hole(struct rrdengine_instance *ctx, struct rrdeng_page_descr *descr, uint8_t remove_dirty);
 extern struct pg_cache_page_index *
         pg_cache_preload(struct rrdengine_instance *ctx, uuid_t *id, usec_t start_time, usec_t end_time);
-extern struct rrdeng_page_cache_descr *
+extern struct rrdeng_page_descr *
         pg_cache_lookup(struct rrdengine_instance *ctx, struct pg_cache_page_index *index, uuid_t *id,
                         usec_t point_in_time);
 extern struct pg_cache_page_index *create_page_index(uuid_t *id);
 extern void init_page_cache(struct rrdengine_instance *ctx);
-extern void pg_cache_add_new_metric_time(struct pg_cache_page_index *page_index, struct rrdeng_page_cache_descr *descr);
+extern void free_page_cache(struct rrdengine_instance *ctx);
+extern void pg_cache_add_new_metric_time(struct pg_cache_page_index *page_index, struct rrdeng_page_descr *descr);
 extern void pg_cache_update_metric_times(struct pg_cache_page_index *page_index);
 
-#endif /* NETDATA_PAGECACHE_H */
\ No newline at end of file
+#endif /* NETDATA_PAGECACHE_H */
diff --git a/database/engine/rrdengine.c b/database/engine/rrdengine.c
index b8e4eba01..0f2dceaa4 100644
--- a/database/engine/rrdengine.c
+++ b/database/engine/rrdengine.c
@@ -3,6 +3,10 @@
 
 #include "rrdengine.h"
 
+rrdeng_stats_t global_io_errors = 0;
+rrdeng_stats_t global_fs_errors = 0;
+rrdeng_stats_t rrdeng_reserved_file_descriptors = 0;
+
 void sanity_check(void)
 {
     /* Magic numbers must fit in the super-blocks */
@@ -27,12 +31,12 @@ void read_extent_cb(uv_fs_t* req)
     struct rrdengine_worker_config* wc = req->loop->data;
     struct rrdengine_instance *ctx = wc->ctx;
     struct extent_io_descriptor *xt_io_descr;
-    struct rrdeng_page_cache_descr *descr;
+    struct rrdeng_page_descr *descr;
+    struct page_cache_descr *pg_cache_descr;
     int ret;
     unsigned i, j, count;
     void *page, *uncompressed_buf = NULL;
     uint32_t payload_length, payload_offset, page_offset, uncompressed_payload_length;
-    struct rrdengine_datafile *datafile;
     /* persistent structures */
     struct rrdeng_df_extent_header *header;
     struct rrdeng_df_extent_trailer *trailer;
@@ -54,9 +58,13 @@ void read_extent_cb(uv_fs_t* req)
     crc = crc32(0L, Z_NULL, 0);
     crc = crc32(crc, xt_io_descr->buf, xt_io_descr->bytes - sizeof(*trailer));
     ret = crc32cmp(trailer->checksum, crc);
-    datafile = xt_io_descr->descr_array[0]->extent->datafile;
-    debug(D_RRDENGINE, "%s: Extent at offset %"PRIu64"(%u) was read from datafile %u-%u. CRC32 check: %s", __func__,
-          xt_io_descr->pos, xt_io_descr->bytes, datafile->tier, datafile->fileno, ret ? "FAILED" : "SUCCEEDED");
+#ifdef NETDATA_INTERNAL_CHECKS
+    {
+        struct rrdengine_datafile *datafile = xt_io_descr->descr_array[0]->extent->datafile;
+        debug(D_RRDENGINE, "%s: Extent at offset %"PRIu64"(%u) was read from datafile %u-%u. CRC32 check: %s", __func__,
+              xt_io_descr->pos, xt_io_descr->bytes, datafile->tier, datafile->fileno, ret ? "FAILED" : "SUCCEEDED");
+    }
+#endif
     if (unlikely(ret)) {
         /* TODO: handle errors */
         exit(UV_EIO);
@@ -97,36 +105,38 @@ void read_extent_cb(uv_fs_t* req)
             (void) memcpy(page, uncompressed_buf + page_offset, descr->page_length);
         }
         pg_cache_replaceQ_insert(ctx, descr);
-        uv_mutex_lock(&descr->mutex);
-        descr->page = page;
-        descr->flags |= RRD_PAGE_POPULATED;
-        descr->flags &= ~RRD_PAGE_READ_PENDING;
+        rrdeng_page_descr_mutex_lock(ctx, descr);
+        pg_cache_descr = descr->pg_cache_descr;
+        pg_cache_descr->page = page;
+        pg_cache_descr->flags |= RRD_PAGE_POPULATED;
+        pg_cache_descr->flags &= ~RRD_PAGE_READ_PENDING;
         debug(D_RRDENGINE, "%s: Waking up waiters.", __func__);
         if (xt_io_descr->release_descr) {
             pg_cache_put_unsafe(descr);
         } else {
             pg_cache_wake_up_waiters_unsafe(descr);
         }
-        uv_mutex_unlock(&descr->mutex);
+        rrdeng_page_descr_mutex_unlock(ctx, descr);
     }
     if (RRD_NO_COMPRESSION != header->compression_algorithm) {
-        free(uncompressed_buf);
+        freez(uncompressed_buf);
     }
     if (xt_io_descr->completion)
         complete(xt_io_descr->completion);
 cleanup:
     uv_fs_req_cleanup(req);
     free(xt_io_descr->buf);
-    free(xt_io_descr);
+    freez(xt_io_descr);
 }
 
 
 static void do_read_extent(struct rrdengine_worker_config* wc,
-                           struct rrdeng_page_cache_descr **descr,
+                           struct rrdeng_page_descr **descr,
                            unsigned count,
                            uint8_t release_descr)
 {
     struct rrdengine_instance *ctx = wc->ctx;
+    struct page_cache_descr *pg_cache_descr;
     int ret;
     unsigned i, size_bytes, pos, real_io_size;
 //    uint32_t payload_length;
@@ -141,14 +151,15 @@ static void do_read_extent(struct rrdengine_worker_config* wc,
     ret = posix_memalign((void *)&xt_io_descr->buf, RRDFILE_ALIGNMENT, ALIGN_BYTES_CEILING(size_bytes));
     if (unlikely(ret)) {
         fatal("posix_memalign:%s", strerror(ret));
-        /* free(xt_io_descr);
+        /* freez(xt_io_descr);
         return;*/
     }
     for (i = 0 ; i < count; ++i) {
-        uv_mutex_lock(&descr[i]->mutex);
-        descr[i]->flags |= RRD_PAGE_READ_PENDING;
+        rrdeng_page_descr_mutex_lock(ctx, descr[i]);
+        pg_cache_descr = descr[i]->pg_cache_descr;
+        pg_cache_descr->flags |= RRD_PAGE_READ_PENDING;
 //        payload_length = descr[i]->page_length;
-        uv_mutex_unlock(&descr[i]->mutex);
+        rrdeng_page_descr_mutex_unlock(ctx, descr[i]);
 
         xt_io_descr->descr_array[i] = descr[i];
     }
@@ -227,8 +238,8 @@ void flush_pages_cb(uv_fs_t* req)
     struct rrdengine_instance *ctx = wc->ctx;
     struct page_cache *pg_cache = &ctx->pg_cache;
     struct extent_io_descriptor *xt_io_descr;
-    struct rrdeng_page_cache_descr *descr;
-    struct rrdengine_datafile *datafile;
+    struct rrdeng_page_descr *descr;
+    struct page_cache_descr *pg_cache_descr;
     int ret;
     unsigned i, count;
     Word_t commit_id;
@@ -238,10 +249,13 @@ void flush_pages_cb(uv_fs_t* req)
         error("%s: uv_fs_write: %s", __func__, uv_strerror((int)req->result));
         goto cleanup;
     }
-    datafile = xt_io_descr->descr_array[0]->extent->datafile;
-    debug(D_RRDENGINE, "%s: Extent at offset %"PRIu64"(%u) was written to datafile %u-%u. Waking up waiters.",
-          __func__, xt_io_descr->pos, xt_io_descr->bytes, datafile->tier, datafile->fileno);
-
+#ifdef NETDATA_INTERNAL_CHECKS
+    {
+        struct rrdengine_datafile *datafile = xt_io_descr->descr_array[0]->extent->datafile;
+        debug(D_RRDENGINE, "%s: Extent at offset %"PRIu64"(%u) was written to datafile %u-%u. Waking up waiters.",
+              __func__, xt_io_descr->pos, xt_io_descr->bytes, datafile->tier, datafile->fileno);
+    }
+#endif
     count = xt_io_descr->descr_count;
     for (i = 0 ; i < count ; ++i) {
         /* care, we don't hold the descriptor mutex */
@@ -256,18 +270,19 @@ void flush_pages_cb(uv_fs_t* req)
 
         pg_cache_replaceQ_insert(ctx, descr);
 
-        uv_mutex_lock(&descr->mutex);
-        descr->flags &= ~(RRD_PAGE_DIRTY | RRD_PAGE_WRITE_PENDING);
+        rrdeng_page_descr_mutex_lock(ctx, descr);
+        pg_cache_descr = descr->pg_cache_descr;
+        pg_cache_descr->flags &= ~(RRD_PAGE_DIRTY | RRD_PAGE_WRITE_PENDING);
         /* wake up waiters, care no reference being held */
         pg_cache_wake_up_waiters_unsafe(descr);
-        uv_mutex_unlock(&descr->mutex);
+        rrdeng_page_descr_mutex_unlock(ctx, descr);
     }
     if (xt_io_descr->completion)
         complete(xt_io_descr->completion);
 cleanup:
     uv_fs_req_cleanup(req);
     free(xt_io_descr->buf);
-    free(xt_io_descr);
+    freez(xt_io_descr);
 }
 
 /*
@@ -283,7 +298,8 @@ static int do_flush_pages(struct rrdengine_worker_config* wc, int force, struct
     int compressed_size, max_compressed_size = 0;
     unsigned i, count, size_bytes, pos, real_io_size;
     uint32_t uncompressed_payload_length, payload_offset;
-    struct rrdeng_page_cache_descr *descr, *eligible_pages[MAX_PAGES_PER_EXTENT];
+    struct rrdeng_page_descr *descr, *eligible_pages[MAX_PAGES_PER_EXTENT];
+    struct page_cache_descr *pg_cache_descr;
     struct extent_io_descriptor *xt_io_descr;
     void *compressed_buf = NULL;
     Word_t descr_commit_idx_array[MAX_PAGES_PER_EXTENT];
@@ -311,15 +327,16 @@ static int do_flush_pages(struct rrdengine_worker_config* wc, int force, struct
          descr = unlikely(NULL == PValue) ? NULL : *PValue) {
         assert(0 != descr->page_length);
 
-        uv_mutex_lock(&descr->mutex);
-        if (!(descr->flags & RRD_PAGE_WRITE_PENDING)) {
+        rrdeng_page_descr_mutex_lock(ctx, descr);
+        pg_cache_descr = descr->pg_cache_descr;
+        if (!(pg_cache_descr->flags & RRD_PAGE_WRITE_PENDING)) {
             /* care, no reference being held */
-            descr->flags |= RRD_PAGE_WRITE_PENDING;
+            pg_cache_descr->flags |= RRD_PAGE_WRITE_PENDING;
             uncompressed_payload_length += descr->page_length;
             descr_commit_idx_array[count] = Index;
             eligible_pages[count++] = descr;
         }
-        uv_mutex_unlock(&descr->mutex);
+        rrdeng_page_descr_mutex_unlock(ctx, descr);
     }
     uv_rwlock_rdunlock(&pg_cache->commited_page_index.lock);
 
@@ -345,9 +362,9 @@ static int do_flush_pages(struct rrdengine_worker_config* wc, int force, struct
     ret = posix_memalign((void *)&xt_io_descr->buf, RRDFILE_ALIGNMENT, ALIGN_BYTES_CEILING(size_bytes));
     if (unlikely(ret)) {
         fatal("posix_memalign:%s", strerror(ret));
-        /* free(xt_io_descr);*/
+        /* freez(xt_io_descr);*/
     }
-    (void) memcpy(xt_io_descr->descr_array, eligible_pages, sizeof(struct rrdeng_page_cache_descr *) * count);
+    (void) memcpy(xt_io_descr->descr_array, eligible_pages, sizeof(struct rrdeng_page_descr *) * count);
     xt_io_descr->descr_count = count;
 
     pos = 0;
@@ -378,7 +395,7 @@ static int do_flush_pages(struct rrdengine_worker_config* wc, int force, struct
     for (i = 0 ; i < count ; ++i) {
         descr = xt_io_descr->descr_array[i];
         /* care, we don't hold the descriptor mutex */
-        (void) memcpy(xt_io_descr->buf + pos, descr->page, descr->page_length);
+        (void) memcpy(xt_io_descr->buf + pos, descr->pg_cache_descr->page, descr->page_length);
         descr->extent = extent;
         extent->pages[i] = descr;
 
@@ -397,7 +414,7 @@ static int do_flush_pages(struct rrdengine_worker_config* wc, int force, struct
         ctx->stats.after_compress_bytes += compressed_size;
         debug(D_RRDENGINE, "LZ4 compressed %"PRIu32" bytes to %d bytes.", uncompressed_payload_length, compressed_size);
         (void) memcpy(xt_io_descr->buf + payload_offset, compressed_buf, compressed_size);
-        free(compressed_buf);
+        freez(compressed_buf);
         size_bytes = payload_offset + compressed_size + sizeof(*trailer);
         header->payload_length = compressed_size;
         break;
@@ -435,23 +452,36 @@ static void after_delete_old_data(uv_work_t *req, int status)
     struct rrdengine_worker_config* wc = &ctx->worker_config;
     struct rrdengine_datafile *datafile;
     struct rrdengine_journalfile *journalfile;
-    unsigned bytes;
+    unsigned deleted_bytes, journalfile_bytes, datafile_bytes;
+    int ret;
+    char path[RRDENG_PATH_MAX];
 
     (void)status;
     datafile = ctx->datafiles.first;
     journalfile = datafile->journalfile;
-    bytes = datafile->pos + journalfile->pos;
+    datafile_bytes = datafile->pos;
+    journalfile_bytes = journalfile->pos;
+    deleted_bytes = 0;
 
+    info("Deleting data and journal file pair.");
     datafile_list_delete(ctx, datafile);
-    destroy_journal_file(journalfile, datafile);
-    destroy_data_file(datafile);
-    info("Deleted data file \""DATAFILE_PREFIX RRDENG_FILE_NUMBER_PRINT_TMPL DATAFILE_EXTENSION"\".",
-         datafile->tier, datafile->fileno);
-    free(journalfile);
-    free(datafile);
+    ret = destroy_journal_file(journalfile, datafile);
+    if (!ret) {
+        generate_journalfilepath(datafile, path, sizeof(path));
+        info("Deleted journal file \"%s\".", path);
+        deleted_bytes += journalfile_bytes;
+    }
+    ret = destroy_data_file(datafile);
+    if (!ret) {
+        generate_datafilepath(datafile, path, sizeof(path));
+        info("Deleted data file \"%s\".", path);
+        deleted_bytes += datafile_bytes;
+    }
+    freez(journalfile);
+    freez(datafile);
 
-    ctx->disk_space -= bytes;
-    info("Reclaimed %u bytes of disk space.", bytes);
+    ctx->disk_space -= deleted_bytes;
+    info("Reclaimed %u bytes of disk space.", deleted_bytes);
 
     /* unfreeze command processing */
     wc->now_deleting.data = NULL;
@@ -464,7 +494,7 @@ static void delete_old_data(uv_work_t *req)
     struct rrdengine_instance *ctx = req->data;
     struct rrdengine_datafile *datafile;
     struct extent_info *extent, *next;
-    struct rrdeng_page_cache_descr *descr;
+    struct rrdeng_page_descr *descr;
     unsigned count, i;
 
     /* Safe to use since it will be deleted after we are done */
@@ -474,10 +504,10 @@ static void delete_old_data(uv_work_t *req)
         count = extent->number_of_pages;
         for (i = 0 ; i < count ; ++i) {
             descr = extent->pages[i];
-            pg_cache_punch_hole(ctx, descr);
+            pg_cache_punch_hole(ctx, descr, 0);
         }
         next = extent->next;
-        free(extent);
+        freez(extent);
     }
 }
 
@@ -487,6 +517,7 @@ void rrdeng_test_quota(struct rrdengine_worker_config* wc)
     struct rrdengine_datafile *datafile;
     unsigned current_size, target_size;
     uint8_t out_of_space, only_one_datafile;
+    int ret;
 
     out_of_space = 0;
     if (unlikely(ctx->disk_space > ctx->max_disk_space)) {
@@ -501,7 +532,10 @@ void rrdeng_test_quota(struct rrdengine_worker_config* wc)
     if (unlikely(current_size >= target_size || (out_of_space && only_one_datafile))) {
         /* Finalize data and journal file and create a new pair */
         wal_flush_transaction_buffer(wc);
-        create_new_datafile_pair(ctx, 1, datafile->fileno + 1);
+        ret = create_new_datafile_pair(ctx, 1, ctx->last_fileno + 1);
+        if (likely(!ret)) {
+            ++ctx->last_fileno;
+        }
     }
     if (unlikely(out_of_space)) {
         /* delete old data */
@@ -509,18 +543,30 @@ void rrdeng_test_quota(struct rrdengine_worker_config* wc)
             /* already deleting data */
             return;
         }
-        info("Deleting data file \""DATAFILE_PREFIX RRDENG_FILE_NUMBER_PRINT_TMPL DATAFILE_EXTENSION"\".",
-             ctx->datafiles.first->tier, ctx->datafiles.first->fileno);
+        if (NULL == ctx->datafiles.first->next) {
+            error("Cannot delete data file \"%s/"DATAFILE_PREFIX RRDENG_FILE_NUMBER_PRINT_TMPL DATAFILE_EXTENSION"\""
+                 " to reclaim space, there are no other file pairs left.",
+                 ctx->dbfiles_path, ctx->datafiles.first->tier, ctx->datafiles.first->fileno);
+            return;
+        }
+        info("Deleting data file \"%s/"DATAFILE_PREFIX RRDENG_FILE_NUMBER_PRINT_TMPL DATAFILE_EXTENSION"\".",
+             ctx->dbfiles_path, ctx->datafiles.first->tier, ctx->datafiles.first->fileno);
         wc->now_deleting.data = ctx;
-        uv_queue_work(wc->loop, &wc->now_deleting, delete_old_data, after_delete_old_data);
+        assert(0 == uv_queue_work(wc->loop, &wc->now_deleting, delete_old_data, after_delete_old_data));
     }
 }
 
+/* return 0 on success */
 int init_rrd_files(struct rrdengine_instance *ctx)
 {
     return init_data_files(ctx);
 }
 
+void finalize_rrd_files(struct rrdengine_instance *ctx)
+{
+    return finalize_data_files(ctx);
+}
+
 void rrdeng_init_cmd_queue(struct rrdengine_worker_config* wc)
 {
     wc->cmd_queue.head = wc->cmd_queue.tail = 0;
@@ -588,7 +634,6 @@ void async_cb(uv_async_t *handle)
 void timer_cb(uv_timer_t* handle)
 {
     struct rrdengine_worker_config* wc = handle->data;
-    struct rrdengine_instance *ctx = wc->ctx;
 
     uv_stop(handle->loop);
     uv_update_time(handle->loop);
@@ -608,7 +653,7 @@ void timer_cb(uv_timer_t* handle)
 #ifdef NETDATA_INTERNAL_CHECKS
     {
         char buf[4096];
-        debug(D_RRDENGINE, "%s", get_rrdeng_statistics(ctx, buf, sizeof(buf)));
+        debug(D_RRDENGINE, "%s", get_rrdeng_statistics(wc->ctx, buf, sizeof(buf)));
     }
 #endif
 }
@@ -623,7 +668,7 @@ void rrdeng_worker(void* arg)
     struct rrdengine_worker_config* wc = arg;
     struct rrdengine_instance *ctx = wc->ctx;
     uv_loop_t* loop;
-    int shutdown;
+    int shutdown, ret;
     enum rrdeng_opcode opcode;
     uv_timer_t timer_req;
     struct rrdeng_cmd cmd;
@@ -631,22 +676,35 @@ void rrdeng_worker(void* arg)
     rrdeng_init_cmd_queue(wc);
 
     loop = wc->loop = mallocz(sizeof(uv_loop_t));
-    uv_loop_init(loop);
+    ret = uv_loop_init(loop);
+    if (ret) {
+        error("uv_loop_init(): %s", uv_strerror(ret));
+        goto error_after_loop_init;
+    }
     loop->data = wc;
 
-    uv_async_init(wc->loop, &wc->async, async_cb);
+    ret = uv_async_init(wc->loop, &wc->async, async_cb);
+    if (ret) {
+        error("uv_async_init(): %s", uv_strerror(ret));
+        goto error_after_async_init;
+    }
     wc->async.data = wc;
 
     wc->now_deleting.data = NULL;
 
     /* dirty page flushing timer */
-    uv_timer_init(loop, &timer_req);
+    ret = uv_timer_init(loop, &timer_req);
+    if (ret) {
+        error("uv_timer_init(): %s", uv_strerror(ret));
+        goto error_after_timer_init;
+    }
     timer_req.data = wc;
 
+    wc->error = 0;
     /* wake up initialization thread */
     complete(&ctx->rrdengine_completion);
 
-    uv_timer_start(&timer_req, timer_cb, TIMER_PERIOD_MS, TIMER_PERIOD_MS);
+    assert(0 == uv_timer_start(&timer_req, timer_cb, TIMER_PERIOD_MS, TIMER_PERIOD_MS));
     shutdown = 0;
     while (shutdown == 0 || uv_loop_alive(loop)) {
         uv_run(loop, UV_RUN_DEFAULT);
@@ -661,12 +719,6 @@ void rrdeng_worker(void* arg)
                 break;
             case RRDENG_SHUTDOWN:
                 shutdown = 1;
-                if (unlikely(wc->now_deleting.data)) {
-                    /* postpone shutdown until after deletion */
-                    info("Postponing shutting RRD engine event loop down until after datafile deletion is finished.");
-                    rrdeng_enq_cmd(wc, &cmd);
-                    break;
-                }
                 /*
                  * uv_async_send after uv_close does not seem to crash in linux at the moment,
                  * it is however undocumented behaviour and we need to be aware if this becomes
@@ -675,10 +727,6 @@ void rrdeng_worker(void* arg)
                 uv_close((uv_handle_t *)&wc->async, NULL);
                 assert(0 == uv_timer_stop(&timer_req));
                 uv_close((uv_handle_t *)&timer_req, NULL);
-                info("Shutting down RRD engine event loop.");
-                while (do_flush_pages(wc, 1, NULL)) {
-                    ; /* Force flushing of all commited pages. */
-                }
                 break;
             case RRDENG_READ_PAGE:
                 do_read_extent(wc, &cmd.read_page.page_cache_descr, 1, 0);
@@ -690,14 +738,14 @@ void rrdeng_worker(void* arg)
                 do_commit_transaction(wc, STORE_DATA, NULL);
                 break;
             case RRDENG_FLUSH_PAGES: {
-                unsigned total_bytes, bytes_written;
+                unsigned bytes_written;
 
                 /* First I/O should be enough to call completion */
                 bytes_written = do_flush_pages(wc, 1, cmd.completion);
-                for (total_bytes =  bytes_written ;
-                     bytes_written && (total_bytes < DATAFILE_IDEAL_IO_SIZE) ;
-                     total_bytes += bytes_written) {
-                    bytes_written = do_flush_pages(wc, 1, NULL);
+                if (bytes_written) {
+                    while (do_flush_pages(wc, 1, NULL)) {
+                        ; /* Force flushing of all commited pages. */
+                    }
                 }
                 break;
             }
@@ -708,6 +756,13 @@ void rrdeng_worker(void* arg)
         } while (opcode != RRDENG_NOOP);
     }
     /* cleanup operations of the event loop */
+    if (unlikely(wc->now_deleting.data)) {
+        info("Postponing shutting RRD engine event loop down until after datafile deletion is finished.");
+    }
+    info("Shutting down RRD engine event loop.");
+    while (do_flush_pages(wc, 1, NULL)) {
+        ; /* Force flushing of all commited pages. */
+    }
     wal_flush_transaction_buffer(wc);
     uv_run(loop, UV_RUN_DEFAULT);
 
@@ -716,7 +771,20 @@ void rrdeng_worker(void* arg)
     uv_cond_destroy(&wc->cmd_cond);
 /*  uv_mutex_destroy(&wc->cmd_mutex); */
     assert(0 == uv_loop_close(loop));
-    free(loop);
+    freez(loop);
+
+    return;
+
+error_after_timer_init:
+    uv_close((uv_handle_t *)&wc->async, NULL);
+error_after_async_init:
+    assert(0 == uv_loop_close(loop));
+error_after_loop_init:
+    freez(loop);
+
+    wc->error = UV_EAGAIN;
+    /* wake up initialization thread */
+    complete(&ctx->rrdengine_completion);
 }
 
 
@@ -726,19 +794,19 @@ static void basic_functional_test(struct rrdengine_instance *ctx)
     int i, j, failed_validations;
     uuid_t uuid[NR_PAGES];
     void *buf;
-    struct rrdeng_page_cache_descr *handle[NR_PAGES];
-    char uuid_str[37];
-    char backup[NR_PAGES][37 * 100]; /* backup storage for page data verification */
+    struct rrdeng_page_descr *handle[NR_PAGES];
+    char uuid_str[UUID_STR_LEN];
+    char backup[NR_PAGES][UUID_STR_LEN * 100]; /* backup storage for page data verification */
 
     for (i = 0 ; i < NR_PAGES ; ++i) {
         uuid_generate(uuid[i]);
         uuid_unparse_lower(uuid[i], uuid_str);
 //      fprintf(stderr, "Generated uuid[%d]=%s\n", i, uuid_str);
-        buf = rrdeng_create_page(&uuid[i], &handle[i]);
+        buf = rrdeng_create_page(ctx, &uuid[i], &handle[i]);
         /* Each page contains 10 times its own UUID stringified */
         for (j = 0 ; j < 100 ; ++j) {
-            strcpy(buf + 37 * j, uuid_str);
-            strcpy(backup[i] + 37 * j, uuid_str);
+            strcpy(buf + UUID_STR_LEN * j, uuid_str);
+            strcpy(backup[i] + UUID_STR_LEN * j, uuid_str);
         }
         rrdeng_commit_page(ctx, handle[i], (Word_t)i);
     }
@@ -750,7 +818,7 @@ static void basic_functional_test(struct rrdengine_instance *ctx)
             ++failed_validations;
             fprintf(stderr, "Page %d was LOST.\n", i);
         }
-        if (memcmp(backup[i], buf, 37 * 100)) {
+        if (memcmp(backup[i], buf, UUID_STR_LEN * 100)) {
             ++failed_validations;
             fprintf(stderr, "Page %d data comparison with backup FAILED validation.\n", i);
         }
diff --git a/database/engine/rrdengine.h b/database/engine/rrdengine.h
index 141bb9c63..6f6a6f8ff 100644
--- a/database/engine/rrdengine.h
+++ b/database/engine/rrdengine.h
@@ -22,6 +22,7 @@
 #include "journalfile.h"
 #include "rrdengineapi.h"
 #include "pagecache.h"
+#include "rrdenglocking.h"
 
 #ifdef NETDATA_RRD_INTERNALS
 
@@ -59,10 +60,10 @@ struct rrdeng_cmd {
     enum rrdeng_opcode opcode;
     union {
         struct rrdeng_read_page {
-            struct rrdeng_page_cache_descr *page_cache_descr;
+            struct rrdeng_page_descr *page_cache_descr;
         } read_page;
         struct rrdeng_read_extent {
-            struct rrdeng_page_cache_descr *page_cache_descr[MAX_PAGES_PER_EXTENT];
+            struct rrdeng_page_descr *page_cache_descr[MAX_PAGES_PER_EXTENT];
             int page_count;
         } read_extent;
         struct completion *completion;
@@ -85,7 +86,7 @@ struct extent_io_descriptor {
     struct completion *completion;
     unsigned descr_count;
     int release_descr;
-    struct rrdeng_page_cache_descr *descr_array[MAX_PAGES_PER_EXTENT];
+    struct rrdeng_page_descr *descr_array[MAX_PAGES_PER_EXTENT];
     Word_t descr_commit_idx_array[MAX_PAGES_PER_EXTENT];
 };
 
@@ -111,6 +112,8 @@ struct rrdengine_worker_config {
     uv_cond_t cmd_cond;
     volatile unsigned queue_size;
     struct rrdeng_cmdqueue cmd_queue;
+
+    int error;
 };
 
 /*
@@ -142,10 +145,19 @@ struct rrdengine_statistics {
     rrdeng_stats_t datafile_deletions;
     rrdeng_stats_t journalfile_creations;
     rrdeng_stats_t journalfile_deletions;
+    rrdeng_stats_t page_cache_descriptors;
+    rrdeng_stats_t io_errors;
+    rrdeng_stats_t fs_errors;
 };
 
+/* I/O errors global counter */
+extern rrdeng_stats_t global_io_errors;
+/* File-System errors global counter */
+extern rrdeng_stats_t global_fs_errors;
+/* number of File-Descriptors that have been reserved by dbengine */
+extern rrdeng_stats_t rrdeng_reserved_file_descriptors;
+
 struct rrdengine_instance {
-    rrdengine_state_t rrdengine_state;
     struct rrdengine_worker_config worker_config;
     struct completion rrdengine_completion;
     struct page_cache pg_cache;
@@ -155,6 +167,7 @@ struct rrdengine_instance {
     char dbfiles_path[FILENAME_MAX+1];
     uint64_t disk_space;
     uint64_t max_disk_space;
+    unsigned last_fileno; /* newest index of datafile and journalfile */
     unsigned long max_cache_pages;
     unsigned long cache_pages_low_watermark;
 
@@ -163,6 +176,7 @@ struct rrdengine_instance {
 
 extern void sanity_check(void);
 extern int init_rrd_files(struct rrdengine_instance *ctx);
+extern void finalize_rrd_files(struct rrdengine_instance *ctx);
 extern void rrdeng_test_quota(struct rrdengine_worker_config* wc);
 extern void rrdeng_worker(void* arg);
 extern void rrdeng_enq_cmd(struct rrdengine_worker_config* wc, struct rrdeng_cmd *cmd);
diff --git a/database/engine/rrdengineapi.c b/database/engine/rrdengineapi.c
index a4e711554..a87ce6d64 100644
--- a/database/engine/rrdengineapi.c
+++ b/database/engine/rrdengineapi.c
@@ -41,6 +41,7 @@ void rrdeng_store_metric_init(RRDDIM *rd)
 
     handle->descr = NULL;
     handle->prev_descr = NULL;
+    handle->unaligned_page = 0;
 
     uv_rwlock_rdlock(&pg_cache->metrics_index.lock);
     PValue = JudyHSGet(pg_cache->metrics_index.JudyHS_array, &temp_id, sizeof(uuid_t));
@@ -54,59 +55,140 @@ void rrdeng_store_metric_init(RRDDIM *rd)
         PValue = JudyHSIns(&pg_cache->metrics_index.JudyHS_array, &temp_id, sizeof(uuid_t), PJE0);
         assert(NULL == *PValue); /* TODO: figure out concurrency model */
         *PValue = page_index = create_page_index(&temp_id);
+        page_index->prev = pg_cache->metrics_index.last_page_index;
+        pg_cache->metrics_index.last_page_index = page_index;
         uv_rwlock_wrunlock(&pg_cache->metrics_index.lock);
     }
     rd->state->rrdeng_uuid = &page_index->id;
     handle->page_index = page_index;
 }
 
+/* The page must be populated and referenced */
+static int page_has_only_empty_metrics(struct rrdeng_page_descr *descr)
+{
+    unsigned i;
+    uint8_t has_only_empty_metrics = 1;
+    storage_number *page;
+
+    page = descr->pg_cache_descr->page;
+    for (i = 0 ; i < descr->page_length / sizeof(storage_number); ++i) {
+        if (SN_EMPTY_SLOT != page[i]) {
+            has_only_empty_metrics = 0;
+            break;
+        }
+    }
+    return has_only_empty_metrics;
+}
+
+void rrdeng_store_metric_flush_current_page(RRDDIM *rd)
+{
+    struct rrdeng_collect_handle *handle;
+    struct rrdengine_instance *ctx;
+    struct rrdeng_page_descr *descr;
+
+    handle = &rd->state->handle.rrdeng;
+    ctx = handle->ctx;
+    descr = handle->descr;
+    if (unlikely(NULL == descr)) {
+        return;
+    }
+    if (likely(descr->page_length)) {
+        int ret, page_is_empty;
+
+#ifdef NETDATA_INTERNAL_CHECKS
+        rrd_stat_atomic_add(&ctx->stats.metric_API_producers, -1);
+#endif
+        if (handle->prev_descr) {
+            /* unpin old second page */
+            pg_cache_put(ctx, handle->prev_descr);
+        }
+        page_is_empty = page_has_only_empty_metrics(descr);
+        if (page_is_empty) {
+            debug(D_RRDENGINE, "Page has empty metrics only, deleting:");
+            if (unlikely(debug_flags & D_RRDENGINE))
+                print_page_cache_descr(descr);
+            pg_cache_put(ctx, descr);
+            pg_cache_punch_hole(ctx, descr, 1);
+            handle->prev_descr = NULL;
+        } else {
+            /* added 1 extra reference to keep 2 dirty pages pinned per metric, expected refcnt = 2 */
+            rrdeng_page_descr_mutex_lock(ctx, descr);
+            ret = pg_cache_try_get_unsafe(descr, 0);
+            rrdeng_page_descr_mutex_unlock(ctx, descr);
+            assert (1 == ret);
+
+            rrdeng_commit_page(ctx, descr, handle->page_correlation_id);
+            handle->prev_descr = descr;
+        }
+    } else {
+        freez(descr->pg_cache_descr->page);
+        rrdeng_destroy_pg_cache_descr(ctx, descr->pg_cache_descr);
+        freez(descr);
+    }
+    handle->descr = NULL;
+}
+
 void rrdeng_store_metric_next(RRDDIM *rd, usec_t point_in_time, storage_number number)
 {
     struct rrdeng_collect_handle *handle;
     struct rrdengine_instance *ctx;
     struct page_cache *pg_cache;
-    struct rrdeng_page_cache_descr *descr;
+    struct rrdeng_page_descr *descr;
     storage_number *page;
+    uint8_t must_flush_unaligned_page = 0, perfect_page_alignment = 0;
 
     handle = &rd->state->handle.rrdeng;
     ctx = handle->ctx;
     pg_cache = &ctx->pg_cache;
     descr = handle->descr;
-    if (unlikely(NULL == descr || descr->page_length + sizeof(number) > RRDENG_BLOCK_SIZE)) {
-        if (descr) {
-            descr->handle = NULL;
-            if (descr->page_length) {
-#ifdef NETDATA_INTERNAL_CHECKS
-                rrd_stat_atomic_add(&ctx->stats.metric_API_producers, -1);
-#endif
-                /* added 1 extra reference to keep 2 dirty pages pinned per metric, expected refcnt = 2 */
-                ++descr->refcnt;
-                rrdeng_commit_page(ctx, descr, handle->page_correlation_id);
-                if (handle->prev_descr) {
-                    /* unpin old second page */
-                    pg_cache_put(handle->prev_descr);
-                }
-                handle->prev_descr = descr;
-            } else {
-                free(descr->page);
-                free(descr);
-                handle->descr = NULL;
-            }
+
+    if (descr) {
+        /* Make alignment decisions */
+
+        if (descr->page_length == rd->rrdset->rrddim_page_alignment) {
+            /* this is the leading dimension that defines chart alignment */
+            perfect_page_alignment = 1;
+        }
+        /* is the metric far enough out of alignment with the others? */
+        if (unlikely(descr->page_length + sizeof(number) < rd->rrdset->rrddim_page_alignment)) {
+            handle->unaligned_page = 1;
+            debug(D_RRDENGINE, "Metric page is not aligned with chart:");
+            if (unlikely(debug_flags & D_RRDENGINE))
+                print_page_cache_descr(descr);
         }
-        page = rrdeng_create_page(&handle->page_index->id, &descr);
+        if (unlikely(handle->unaligned_page &&
+                     /* did the other metrics change page? */
+                     rd->rrdset->rrddim_page_alignment <= sizeof(number))) {
+            debug(D_RRDENGINE, "Flushing unaligned metric page.");
+            must_flush_unaligned_page = 1;
+            handle->unaligned_page = 0;
+        }
+    }
+    if (unlikely(NULL == descr ||
+                 descr->page_length + sizeof(number) > RRDENG_BLOCK_SIZE ||
+                 must_flush_unaligned_page)) {
+        rrdeng_store_metric_flush_current_page(rd);
+
+        page = rrdeng_create_page(ctx, &handle->page_index->id, &descr);
         assert(page);
-        handle->prev_descr = handle->descr;
+
         handle->descr = descr;
-        descr->handle = handle;
+
         uv_rwlock_wrlock(&pg_cache->commited_page_index.lock);
         handle->page_correlation_id = pg_cache->commited_page_index.latest_corr_id++;
         uv_rwlock_wrunlock(&pg_cache->commited_page_index.lock);
-    }
-    page = descr->page;
 
+        if (0 == rd->rrdset->rrddim_page_alignment) {
+            /* this is the leading dimension that defines chart alignment */
+            perfect_page_alignment = 1;
+        }
+    }
+    page = descr->pg_cache_descr->page;
     page[descr->page_length / sizeof(number)] = number;
     descr->end_time = point_in_time;
     descr->page_length += sizeof(number);
+    if (perfect_page_alignment)
+        rd->rrdset->rrddim_page_alignment = descr->page_length;
     if (unlikely(INVALID_TIME == descr->start_time)) {
         descr->start_time = point_in_time;
 
@@ -126,26 +208,13 @@ void rrdeng_store_metric_finalize(RRDDIM *rd)
 {
     struct rrdeng_collect_handle *handle;
     struct rrdengine_instance *ctx;
-    struct rrdeng_page_cache_descr *descr;
 
     handle = &rd->state->handle.rrdeng;
     ctx = handle->ctx;
-    descr = handle->descr;
-    if (descr) {
-        descr->handle = NULL;
-        if (descr->page_length) {
-#ifdef NETDATA_INTERNAL_CHECKS
-            rrd_stat_atomic_add(&ctx->stats.metric_API_producers, -1);
-#endif
-            rrdeng_commit_page(ctx, descr, handle->page_correlation_id);
-            if (handle->prev_descr) {
-                /* unpin old second page */
-                pg_cache_put(handle->prev_descr);
-            }
-        } else {
-            free(descr->page);
-            free(descr);
-        }
+    rrdeng_store_metric_flush_current_page(rd);
+    if (handle->prev_descr) {
+        /* unpin old second page */
+        pg_cache_put(ctx, handle->prev_descr);
     }
 }
 
@@ -174,7 +243,7 @@ storage_number rrdeng_load_metric_next(struct rrddim_query_handle *rrdimm_handle
 {
     struct rrdeng_query_handle *handle;
     struct rrdengine_instance *ctx;
-    struct rrdeng_page_cache_descr *descr;
+    struct rrdeng_page_descr *descr;
     storage_number *page, ret;
     unsigned position;
     usec_t point_in_time;
@@ -198,7 +267,7 @@ storage_number rrdeng_load_metric_next(struct rrddim_query_handle *rrdimm_handle
 #ifdef NETDATA_INTERNAL_CHECKS
             rrd_stat_atomic_add(&ctx->stats.metric_API_consumers, -1);
 #endif
-            pg_cache_put(descr);
+            pg_cache_put(ctx, descr);
             handle->descr = NULL;
         }
         descr = pg_cache_lookup(ctx, handle->page_index, &handle->page_index->id, point_in_time);
@@ -216,7 +285,7 @@ storage_number rrdeng_load_metric_next(struct rrddim_query_handle *rrdimm_handle
         ret = SN_EMPTY_SLOT;
         goto out;
     }
-    page = descr->page;
+    page = descr->pg_cache_descr->page;
     if (unlikely(descr->start_time == descr->end_time)) {
         ret = page[0];
         goto out;
@@ -248,7 +317,7 @@ void rrdeng_load_metric_finalize(struct rrddim_query_handle *rrdimm_handle)
 {
     struct rrdeng_query_handle *handle;
     struct rrdengine_instance *ctx;
-    struct rrdeng_page_cache_descr *descr;
+    struct rrdeng_page_descr *descr;
 
     handle = &rrdimm_handle->rrdeng;
     ctx = handle->ctx;
@@ -257,7 +326,7 @@ void rrdeng_load_metric_finalize(struct rrddim_query_handle *rrdimm_handle)
 #ifdef NETDATA_INTERNAL_CHECKS
         rrd_stat_atomic_add(&ctx->stats.metric_API_consumers, -1);
 #endif
-        pg_cache_put(descr);
+        pg_cache_put(ctx, descr);
     }
 }
 
@@ -283,30 +352,32 @@ time_t rrdeng_metric_oldest_time(RRDDIM *rd)
 }
 
 /* Also gets a reference for the page */
-void *rrdeng_create_page(uuid_t *id, struct rrdeng_page_cache_descr **ret_descr)
+void *rrdeng_create_page(struct rrdengine_instance *ctx, uuid_t *id, struct rrdeng_page_descr **ret_descr)
 {
-    struct rrdeng_page_cache_descr *descr;
+    struct rrdeng_page_descr *descr;
+    struct page_cache_descr *pg_cache_descr;
     void *page;
-    int ret;
-
     /* TODO: check maximum number of pages in page cache limit */
 
-    page = mallocz(RRDENG_BLOCK_SIZE); /*TODO: add page size */
     descr = pg_cache_create_descr();
-    descr->page = page;
     descr->id = id; /* TODO: add page type: metric, log, something? */
-    descr->flags = RRD_PAGE_DIRTY /*| RRD_PAGE_LOCKED */ | RRD_PAGE_POPULATED /* | BEING_COLLECTED */;
-    descr->refcnt = 1;
-
-    debug(D_RRDENGINE, "-----------------\nCreated new page:\n-----------------");
-    if(unlikely(debug_flags & D_RRDENGINE))
+    page = mallocz(RRDENG_BLOCK_SIZE); /*TODO: add page size */
+    rrdeng_page_descr_mutex_lock(ctx, descr);
+    pg_cache_descr = descr->pg_cache_descr;
+    pg_cache_descr->page = page;
+    pg_cache_descr->flags = RRD_PAGE_DIRTY /*| RRD_PAGE_LOCKED */ | RRD_PAGE_POPULATED /* | BEING_COLLECTED */;
+    pg_cache_descr->refcnt = 1;
+
+    debug(D_RRDENGINE, "Created new page:");
+    if (unlikely(debug_flags & D_RRDENGINE))
         print_page_cache_descr(descr);
+    rrdeng_page_descr_mutex_unlock(ctx, descr);
     *ret_descr = descr;
     return page;
 }
 
 /* The page must not be empty */
-void rrdeng_commit_page(struct rrdengine_instance *ctx, struct rrdeng_page_cache_descr *descr,
+void rrdeng_commit_page(struct rrdengine_instance *ctx, struct rrdeng_page_descr *descr,
                         Word_t page_correlation_id)
 {
     struct page_cache *pg_cache = &ctx->pg_cache;
@@ -324,15 +395,16 @@ void rrdeng_commit_page(struct rrdengine_instance *ctx, struct rrdeng_page_cache
     ++pg_cache->commited_page_index.nr_commited_pages;
     uv_rwlock_wrunlock(&pg_cache->commited_page_index.lock);
 
-    pg_cache_put(descr);
+    pg_cache_put(ctx, descr);
 }
 
 /* Gets a reference for the page */
 void *rrdeng_get_latest_page(struct rrdengine_instance *ctx, uuid_t *id, void **handle)
 {
-    struct rrdeng_page_cache_descr *descr;
+    struct rrdeng_page_descr *descr;
+    struct page_cache_descr *pg_cache_descr;
 
-    debug(D_RRDENGINE, "----------------------\nReading existing page:\n----------------------");
+    debug(D_RRDENGINE, "Reading existing page:");
     descr = pg_cache_lookup(ctx, NULL, id, INVALID_TIME);
     if (NULL == descr) {
         *handle = NULL;
@@ -340,16 +412,18 @@ void *rrdeng_get_latest_page(struct rrdengine_instance *ctx, uuid_t *id, void **
         return NULL;
     }
     *handle = descr;
+    pg_cache_descr = descr->pg_cache_descr;
 
-    return descr->page;
+    return pg_cache_descr->page;
 }
 
 /* Gets a reference for the page */
 void *rrdeng_get_page(struct rrdengine_instance *ctx, uuid_t *id, usec_t point_in_time, void **handle)
 {
-    struct rrdeng_page_cache_descr *descr;
+    struct rrdeng_page_descr *descr;
+    struct page_cache_descr *pg_cache_descr;
 
-    debug(D_RRDENGINE, "----------------------\nReading existing page:\n----------------------");
+    debug(D_RRDENGINE, "Reading existing page:");
     descr = pg_cache_lookup(ctx, NULL, id, point_in_time);
     if (NULL == descr) {
         *handle = NULL;
@@ -357,11 +431,18 @@ void *rrdeng_get_page(struct rrdengine_instance *ctx, uuid_t *id, usec_t point_i
         return NULL;
     }
     *handle = descr;
+    pg_cache_descr = descr->pg_cache_descr;
 
-    return descr->page;
+    return pg_cache_descr->page;
 }
 
-void rrdeng_get_27_statistics(struct rrdengine_instance *ctx, unsigned long long *array)
+/*
+ * Gathers Database Engine statistics.
+ * Careful when modifying this function.
+ * You must not change the indices of the statistics or user code will break.
+ * You must not exceed RRDENG_NR_STATS or it will crash.
+ */
+void rrdeng_get_33_statistics(struct rrdengine_instance *ctx, unsigned long long *array)
 {
     struct page_cache *pg_cache = &ctx->pg_cache;
 
@@ -392,24 +473,46 @@ void rrdeng_get_27_statistics(struct rrdengine_instance *ctx, unsigned long long
     array[24] = (uint64_t)ctx->stats.datafile_deletions;
     array[25] = (uint64_t)ctx->stats.journalfile_creations;
     array[26] = (uint64_t)ctx->stats.journalfile_deletions;
+    array[27] = (uint64_t)ctx->stats.page_cache_descriptors;
+    array[28] = (uint64_t)ctx->stats.io_errors;
+    array[29] = (uint64_t)ctx->stats.fs_errors;
+    array[30] = (uint64_t)global_io_errors;
+    array[31] = (uint64_t)global_fs_errors;
+    array[32] = (uint64_t)rrdeng_reserved_file_descriptors;
+    assert(RRDENG_NR_STATS == 33);
 }
 
 /* Releases reference to page */
 void rrdeng_put_page(struct rrdengine_instance *ctx, void *handle)
 {
     (void)ctx;
-    pg_cache_put((struct rrdeng_page_cache_descr *)handle);
+    pg_cache_put(ctx, (struct rrdeng_page_descr *)handle);
 }
 
 /*
- * Returns 0 on success, 1 on error
+ * Returns 0 on success, negative on error
  */
 int rrdeng_init(struct rrdengine_instance **ctxp, char *dbfiles_path, unsigned page_cache_mb, unsigned disk_space_mb)
 {
     struct rrdengine_instance *ctx;
     int error;
+    uint32_t max_open_files;
 
     sanity_check();
+
+    max_open_files = rlimit_nofile.rlim_cur / 4;
+
+    /* reserve RRDENG_FD_BUDGET_PER_INSTANCE file descriptors for this instance */
+    rrd_stat_atomic_add(&rrdeng_reserved_file_descriptors, RRDENG_FD_BUDGET_PER_INSTANCE);
+    if (rrdeng_reserved_file_descriptors > max_open_files) {
+        error("Exceeded the budget of available file descriptors (%u/%u), cannot create new dbengine instance.",
+              (unsigned)rrdeng_reserved_file_descriptors, (unsigned)max_open_files);
+
+        rrd_stat_atomic_add(&global_fs_errors, 1);
+        rrd_stat_atomic_add(&rrdeng_reserved_file_descriptors, -RRDENG_FD_BUDGET_PER_INSTANCE);
+        return UV_EMFILE;
+    }
+
     if (NULL == ctxp) {
         /* for testing */
         ctx = &default_global_ctx;
@@ -417,10 +520,6 @@ int rrdeng_init(struct rrdengine_instance **ctxp, char *dbfiles_path, unsigned p
     } else {
         *ctxp = ctx = callocz(1, sizeof(*ctx));
     }
-    if (ctx->rrdengine_state != RRDENGINE_STATUS_UNINITIALIZED) {
-        return 1;
-    }
-    ctx->rrdengine_state = RRDENGINE_STATUS_INITIALIZING;
     ctx->global_compress_alg = RRD_LZ4;
     if (page_cache_mb < RRDENG_MIN_PAGE_CACHE_SIZE_MB)
         page_cache_mb = RRDENG_MIN_PAGE_CACHE_SIZE_MB;
@@ -439,11 +538,7 @@ int rrdeng_init(struct rrdengine_instance **ctxp, char *dbfiles_path, unsigned p
     init_commit_log(ctx);
     error = init_rrd_files(ctx);
     if (error) {
-        ctx->rrdengine_state = RRDENGINE_STATUS_UNINITIALIZED;
-        if (ctx != &default_global_ctx) {
-            freez(ctx);
-        }
-        return 1;
+        goto error_after_init_rrd_files;
     }
 
     init_completion(&ctx->rrdengine_completion);
@@ -451,9 +546,21 @@ int rrdeng_init(struct rrdengine_instance **ctxp, char *dbfiles_path, unsigned p
     /* wait for worker thread to initialize */
     wait_for_completion(&ctx->rrdengine_completion);
     destroy_completion(&ctx->rrdengine_completion);
-
-    ctx->rrdengine_state = RRDENGINE_STATUS_INITIALIZED;
+    if (ctx->worker_config.error) {
+        goto error_after_rrdeng_worker;
+    }
     return 0;
+
+error_after_rrdeng_worker:
+    finalize_rrd_files(ctx);
+error_after_init_rrd_files:
+    free_page_cache(ctx);
+    if (ctx != &default_global_ctx) {
+        freez(ctx);
+        *ctxp = NULL;
+    }
+    rrd_stat_atomic_add(&rrdeng_reserved_file_descriptors, -RRDENG_FD_BUDGET_PER_INSTANCE);
+    return UV_EIO;
 }
 
 /*
@@ -464,10 +571,6 @@ int rrdeng_exit(struct rrdengine_instance *ctx)
     struct rrdeng_cmd cmd;
 
     if (NULL == ctx) {
-        /* TODO: move to per host basis */
-        ctx = &default_global_ctx;
-    }
-    if (ctx->rrdengine_state != RRDENGINE_STATUS_INITIALIZED) {
         return 1;
     }
 
@@ -477,8 +580,12 @@ int rrdeng_exit(struct rrdengine_instance *ctx)
 
     assert(0 == uv_thread_join(&ctx->worker_config.thread));
 
+    finalize_rrd_files(ctx);
+    free_page_cache(ctx);
+
     if (ctx != &default_global_ctx) {
         freez(ctx);
     }
+    rrd_stat_atomic_add(&rrdeng_reserved_file_descriptors, -RRDENG_FD_BUDGET_PER_INSTANCE);
     return 0;
 }
\ No newline at end of file
diff --git a/database/engine/rrdengineapi.h b/database/engine/rrdengineapi.h
index e76629a4b..e52aabcbe 100644
--- a/database/engine/rrdengineapi.h
+++ b/database/engine/rrdengineapi.h
@@ -7,16 +7,22 @@
 
 #define RRDENG_MIN_PAGE_CACHE_SIZE_MB (32)
 #define RRDENG_MIN_DISK_SPACE_MB (256)
+
+#define RRDENG_NR_STATS (33)
+
+#define RRDENG_FD_BUDGET_PER_INSTANCE (50)
+
 extern int default_rrdeng_page_cache_mb;
 extern int default_rrdeng_disk_quota_mb;
 
-extern void *rrdeng_create_page(uuid_t *id, struct rrdeng_page_cache_descr **ret_descr);
-extern void rrdeng_commit_page(struct rrdengine_instance *ctx, struct rrdeng_page_cache_descr *descr,
+extern void *rrdeng_create_page(struct rrdengine_instance *ctx, uuid_t *id, struct rrdeng_page_descr **ret_descr);
+extern void rrdeng_commit_page(struct rrdengine_instance *ctx, struct rrdeng_page_descr *descr,
                                Word_t page_correlation_id);
 extern void *rrdeng_get_latest_page(struct rrdengine_instance *ctx, uuid_t *id, void **handle);
 extern void *rrdeng_get_page(struct rrdengine_instance *ctx, uuid_t *id, usec_t point_in_time, void **handle);
 extern void rrdeng_put_page(struct rrdengine_instance *ctx, void *handle);
 extern void rrdeng_store_metric_init(RRDDIM *rd);
+extern void rrdeng_store_metric_flush_current_page(RRDDIM *rd);
 extern void rrdeng_store_metric_next(RRDDIM *rd, usec_t point_in_time, storage_number number);
 extern void rrdeng_store_metric_finalize(RRDDIM *rd);
 extern void rrdeng_load_metric_init(RRDDIM *rd, struct rrddim_query_handle *rrdimm_handle,
@@ -26,7 +32,7 @@ extern int rrdeng_load_metric_is_finished(struct rrddim_query_handle *rrdimm_han
 extern void rrdeng_load_metric_finalize(struct rrddim_query_handle *rrdimm_handle);
 extern time_t rrdeng_metric_latest_time(RRDDIM *rd);
 extern time_t rrdeng_metric_oldest_time(RRDDIM *rd);
-extern void rrdeng_get_27_statistics(struct rrdengine_instance *ctx, unsigned long long *array);
+extern void rrdeng_get_33_statistics(struct rrdengine_instance *ctx, unsigned long long *array);
 
 /* must call once before using anything */
 extern int rrdeng_init(struct rrdengine_instance **ctxp, char *dbfiles_path, unsigned page_cache_mb,
diff --git a/database/engine/rrdenginelib.c b/database/engine/rrdenginelib.c
index 25f57ba1b..96504b275 100644
--- a/database/engine/rrdenginelib.c
+++ b/database/engine/rrdenginelib.c
@@ -1,25 +1,52 @@
 // SPDX-License-Identifier: GPL-3.0-or-later
 #include "rrdengine.h"
 
-void print_page_cache_descr(struct rrdeng_page_cache_descr *page_cache_descr)
+#define BUFSIZE (512)
+
+/* Caller must hold descriptor lock */
+void print_page_cache_descr(struct rrdeng_page_descr *descr)
 {
-    char uuid_str[37];
-    char str[512];
+    struct page_cache_descr *pg_cache_descr = descr->pg_cache_descr;
+    char uuid_str[UUID_STR_LEN];
+    char str[BUFSIZE];
     int pos = 0;
 
-    uuid_unparse_lower(*page_cache_descr->id, uuid_str);
-    pos += snprintfz(str, 512 - pos, "page(%p) id=%s\n"
+    uuid_unparse_lower(*descr->id, uuid_str);
+    pos += snprintfz(str, BUFSIZE - pos, "page(%p) id=%s\n"
                                     "--->len:%"PRIu32" time:%"PRIu64"->%"PRIu64" xt_offset:",
-                    page_cache_descr->page, uuid_str,
-                    page_cache_descr->page_length,
-                    (uint64_t)page_cache_descr->start_time,
-                    (uint64_t)page_cache_descr->end_time);
-    if (!page_cache_descr->extent) {
-        pos += snprintfz(str + pos, 512 - pos, "N/A");
+                    pg_cache_descr->page, uuid_str,
+                    descr->page_length,
+                    (uint64_t)descr->start_time,
+                    (uint64_t)descr->end_time);
+    if (!descr->extent) {
+        pos += snprintfz(str + pos, BUFSIZE - pos, "N/A");
+    } else {
+        pos += snprintfz(str + pos, BUFSIZE - pos, "%"PRIu64, descr->extent->offset);
+    }
+
+    snprintfz(str + pos, BUFSIZE - pos, " flags:0x%2.2lX refcnt:%u\n\n", pg_cache_descr->flags, pg_cache_descr->refcnt);
+    debug(D_RRDENGINE, "%s", str);
+}
+
+void print_page_descr(struct rrdeng_page_descr *descr)
+{
+    char uuid_str[UUID_STR_LEN];
+    char str[BUFSIZE];
+    int pos = 0;
+
+    uuid_unparse_lower(*descr->id, uuid_str);
+    pos += snprintfz(str, BUFSIZE - pos, "id=%s\n"
+                                     "--->len:%"PRIu32" time:%"PRIu64"->%"PRIu64" xt_offset:",
+                     uuid_str,
+                     descr->page_length,
+                     (uint64_t)descr->start_time,
+                     (uint64_t)descr->end_time);
+    if (!descr->extent) {
+        pos += snprintfz(str + pos, BUFSIZE - pos, "N/A");
     } else {
-        pos += snprintfz(str + pos, 512 - pos, "%"PRIu64, page_cache_descr->extent->offset);
+        pos += snprintfz(str + pos, BUFSIZE - pos, "%"PRIu64, descr->extent->offset);
     }
-    snprintfz(str + pos, 512 - pos, " flags:0x%2.2lX refcnt:%u\n\n", page_cache_descr->flags, page_cache_descr->refcnt);
+    snprintfz(str + pos, BUFSIZE - pos, "\n\n");
     fputs(str, stderr);
 }
 
@@ -51,6 +78,48 @@ int check_file_properties(uv_file file, uint64_t *file_size, size_t min_size)
     return 0;
 }
 
+/*
+ * Tries to open a file in direct I/O mode, falls back to buffered mode if not possible.
+ * Returns UV error number that is < 0 on failure.
+ * On success sets (*file) to be the uv_file that was opened.
+ */
+int open_file_direct_io(char *path, int flags, uv_file *file)
+{
+    uv_fs_t req;
+    int fd, current_flags, direct;
+
+    for (direct = 1 ; direct >= 0 ; --direct) {
+#ifdef __APPLE__
+        /* Apple OS does not support O_DIRECT */
+        direct = 0;
+#endif
+        current_flags = flags;
+        if (direct) {
+            current_flags |= O_DIRECT;
+        }
+        fd = uv_fs_open(NULL, &req, path, current_flags, S_IRUSR | S_IWUSR, NULL);
+        if (fd < 0) {
+            if ((direct) && (UV_EINVAL == fd)) {
+                error("File \"%s\" does not support direct I/O, falling back to buffered I/O.", path);
+            } else {
+                error("Failed to open file \"%s\".", path);
+                --direct; /* break the loop */
+            }
+        } else {
+            assert(req.result >= 0);
+            *file = req.result;
+#ifdef __APPLE__
+            info("Disabling OS X caching for file \"%s\".", path);
+            fcntl(fd, F_NOCACHE, 1);
+#endif
+            --direct; /* break the loop */
+        }
+        uv_fs_req_cleanup(&req);
+    }
+
+    return fd;
+}
+
 char *get_rrdeng_statistics(struct rrdengine_instance *ctx, char *str, size_t size)
 {
     struct page_cache *pg_cache;
@@ -60,6 +129,7 @@ char *get_rrdeng_statistics(struct rrdengine_instance *ctx, char *str, size_t si
               "metric_API_producers: %ld\n"
               "metric_API_consumers: %ld\n"
               "page_cache_total_pages: %ld\n"
+              "page_cache_descriptors: %ld\n"
               "page_cache_populated_pages: %ld\n"
               "page_cache_commited_pages: %ld\n"
               "page_cache_insertions: %ld\n"
@@ -87,6 +157,7 @@ char *get_rrdeng_statistics(struct rrdengine_instance *ctx, char *str, size_t si
               (long)ctx->stats.metric_API_producers,
               (long)ctx->stats.metric_API_consumers,
               (long)pg_cache->page_descriptors,
+              (long)ctx->stats.page_cache_descriptors,
               (long)pg_cache->populated_pages,
               (long)pg_cache->commited_page_index.nr_commited_pages,
               (long)ctx->stats.pg_cache_insertions,
diff --git a/database/engine/rrdenginelib.h b/database/engine/rrdenginelib.h
index bb6f072bf..36d414e89 100644
--- a/database/engine/rrdenginelib.h
+++ b/database/engine/rrdenginelib.h
@@ -6,11 +6,17 @@
 #include "rrdengine.h"
 
 /* Forward declarations */
-struct rrdeng_page_cache_descr;
+struct rrdeng_page_descr;
 
 #define STR_HELPER(x) #x
 #define STR(x) STR_HELPER(x)
 
+#define BITS_PER_ULONG (sizeof(unsigned long) * 8)
+
+#ifndef UUID_STR_LEN
+#define UUID_STR_LEN (37)
+#endif
+
 /* Taken from linux kernel */
 #define BUILD_BUG_ON(condition) ((void)sizeof(char[1 - 2*!!(condition)]))
 
@@ -25,6 +31,15 @@ typedef uintptr_t rrdeng_stats_t;
 #define rrd_stat_atomic_add(p, n) do {(void) __sync_fetch_and_add(p, n);} while(0)
 #endif
 
+#define RRDENG_PATH_MAX (4096)
+
+/* returns old *ptr value */
+static inline unsigned long ulong_compare_and_swap(volatile unsigned long *ptr,
+                                                   unsigned long oldval, unsigned long newval)
+{
+    return __sync_val_compare_and_swap(ptr, oldval, newval);
+}
+
 #ifndef O_DIRECT
 /* Workaround for OS X */
 #define O_DIRECT (0)
@@ -77,8 +92,10 @@ static inline void crc32set(void *crcp, uLong crc)
     *(uint32_t *)crcp = crc;
 }
 
-extern void print_page_cache_descr(struct rrdeng_page_cache_descr *page_cache_descr);
+extern void print_page_cache_descr(struct rrdeng_page_descr *page_cache_descr);
+extern void print_page_descr(struct rrdeng_page_descr *descr);
 extern int check_file_properties(uv_file file, uint64_t *file_size, size_t min_size);
+extern int open_file_direct_io(char *path, int flags, uv_file *file);
 extern char *get_rrdeng_statistics(struct rrdengine_instance *ctx, char *str, size_t size);
 
 #endif /* NETDATA_RRDENGINELIB_H */
\ No newline at end of file
diff --git a/database/engine/rrdenglocking.c b/database/engine/rrdenglocking.c
new file mode 100644
index 000000000..0eb9019b4
--- /dev/null
+++ b/database/engine/rrdenglocking.c
@@ -0,0 +1,233 @@
+// SPDX-License-Identifier: GPL-3.0-or-later
+#include "rrdengine.h"
+
+struct page_cache_descr *rrdeng_create_pg_cache_descr(struct rrdengine_instance *ctx)
+{
+    struct page_cache_descr *pg_cache_descr;
+
+    pg_cache_descr = mallocz(sizeof(*pg_cache_descr));
+    rrd_stat_atomic_add(&ctx->stats.page_cache_descriptors, 1);
+    pg_cache_descr->page = NULL;
+    pg_cache_descr->flags = 0;
+    pg_cache_descr->prev = pg_cache_descr->next = NULL;
+    pg_cache_descr->refcnt = 0;
+    pg_cache_descr->waiters = 0;
+    assert(0 == uv_cond_init(&pg_cache_descr->cond));
+    assert(0 == uv_mutex_init(&pg_cache_descr->mutex));
+
+    return pg_cache_descr;
+}
+
+void rrdeng_destroy_pg_cache_descr(struct rrdengine_instance *ctx, struct page_cache_descr *pg_cache_descr)
+{
+    uv_cond_destroy(&pg_cache_descr->cond);
+    uv_mutex_destroy(&pg_cache_descr->mutex);
+    freez(pg_cache_descr);
+    rrd_stat_atomic_add(&ctx->stats.page_cache_descriptors, -1);
+}
+
+/* also allocates page cache descriptor if missing */
+void rrdeng_page_descr_mutex_lock(struct rrdengine_instance *ctx, struct rrdeng_page_descr *descr)
+{
+    unsigned long old_state, old_users, new_state, ret_state;
+    struct page_cache_descr *pg_cache_descr = NULL;
+    uint8_t we_locked;
+
+    we_locked = 0;
+    while (1) { /* spin */
+        old_state = descr->pg_cache_descr_state;
+        old_users = old_state >> PG_CACHE_DESCR_SHIFT;
+
+        if (unlikely(we_locked)) {
+            assert(old_state & PG_CACHE_DESCR_LOCKED);
+            new_state = (1 << PG_CACHE_DESCR_SHIFT) | PG_CACHE_DESCR_ALLOCATED;
+            ret_state = ulong_compare_and_swap(&descr->pg_cache_descr_state, old_state, new_state);
+            if (old_state == ret_state) {
+                /* success */
+                break;
+            }
+            continue; /* spin */
+        }
+        if (old_state & PG_CACHE_DESCR_LOCKED) {
+            assert(0 == old_users);
+            continue; /* spin */
+        }
+        if (0 == old_state) {
+            /* no page cache descriptor has been allocated */
+
+            if (NULL == pg_cache_descr) {
+                pg_cache_descr = rrdeng_create_pg_cache_descr(ctx);
+            }
+            new_state = PG_CACHE_DESCR_LOCKED;
+            ret_state = ulong_compare_and_swap(&descr->pg_cache_descr_state, 0, new_state);
+            if (0 == ret_state) {
+                we_locked = 1;
+                descr->pg_cache_descr = pg_cache_descr;
+                pg_cache_descr->descr = descr;
+                pg_cache_descr = NULL; /* make sure we don't free pg_cache_descr */
+                /* retry */
+                continue;
+            }
+            continue; /* spin */
+        }
+        /* page cache descriptor is already allocated */
+        assert(old_state & PG_CACHE_DESCR_ALLOCATED);
+
+        new_state = (old_users + 1) << PG_CACHE_DESCR_SHIFT;
+        new_state |= old_state & PG_CACHE_DESCR_FLAGS_MASK;
+
+        ret_state = ulong_compare_and_swap(&descr->pg_cache_descr_state, old_state, new_state);
+        if (old_state == ret_state) {
+            /* success */
+            break;
+        }
+        /* spin */
+    }
+
+    if (pg_cache_descr) {
+        rrdeng_destroy_pg_cache_descr(ctx, pg_cache_descr);
+    }
+    pg_cache_descr = descr->pg_cache_descr;
+    uv_mutex_lock(&pg_cache_descr->mutex);
+}
+
+void rrdeng_page_descr_mutex_unlock(struct rrdengine_instance *ctx, struct rrdeng_page_descr *descr)
+{
+    unsigned long old_state, new_state, ret_state, old_users;
+    struct page_cache_descr *pg_cache_descr;
+    uint8_t we_locked;
+
+    uv_mutex_unlock(&descr->pg_cache_descr->mutex);
+
+    we_locked = 0;
+    while (1) { /* spin */
+        old_state = descr->pg_cache_descr_state;
+        old_users = old_state >> PG_CACHE_DESCR_SHIFT;
+
+        if (unlikely(we_locked)) {
+            assert(0 == old_users);
+
+            ret_state = ulong_compare_and_swap(&descr->pg_cache_descr_state, old_state, 0);
+            if (old_state == ret_state) {
+                /* success */
+                break;
+            }
+            continue; /* spin */
+        }
+        if (old_state & PG_CACHE_DESCR_LOCKED) {
+            assert(0 == old_users);
+            continue; /* spin */
+        }
+        assert(old_state & PG_CACHE_DESCR_ALLOCATED);
+        pg_cache_descr = descr->pg_cache_descr;
+        /* caller is the only page cache descriptor user and there are no pending references on the page */
+        if ((old_state & PG_CACHE_DESCR_DESTROY) && (1 == old_users) &&
+            !pg_cache_descr->flags && !pg_cache_descr->refcnt) {
+            new_state = PG_CACHE_DESCR_LOCKED;
+            ret_state = ulong_compare_and_swap(&descr->pg_cache_descr_state, old_state, new_state);
+            if (old_state == ret_state) {
+                we_locked = 1;
+                rrdeng_destroy_pg_cache_descr(ctx, pg_cache_descr);
+                /* retry */
+                continue;
+            }
+            continue; /* spin */
+        }
+        assert(old_users > 0);
+        new_state = (old_users - 1) << PG_CACHE_DESCR_SHIFT;
+        new_state |= old_state & PG_CACHE_DESCR_FLAGS_MASK;
+
+        ret_state = ulong_compare_and_swap(&descr->pg_cache_descr_state, old_state, new_state);
+        if (old_state == ret_state) {
+            /* success */
+            break;
+        }
+        /* spin */
+    }
+
+}
+
+/*
+ * Tries to deallocate page cache descriptor. If it fails, it postpones deallocation by setting the
+ * PG_CACHE_DESCR_DESTROY flag which will be eventually cleared by a different context after doing
+ * the deallocation.
+ */
+void rrdeng_try_deallocate_pg_cache_descr(struct rrdengine_instance *ctx, struct rrdeng_page_descr *descr)
+{
+    unsigned long old_state, new_state, ret_state, old_users;
+    struct page_cache_descr *pg_cache_descr;
+    uint8_t just_locked, we_freed, must_unlock;
+
+    just_locked = 0;
+    we_freed = 0;
+    must_unlock = 0;
+    while (1) { /* spin */
+        old_state = descr->pg_cache_descr_state;
+        old_users = old_state >> PG_CACHE_DESCR_SHIFT;
+
+        if (unlikely(just_locked)) {
+            assert(0 == old_users);
+
+            must_unlock = 1;
+            just_locked = 0;
+            /* Try deallocate if there are no pending references on the page */
+            if (!pg_cache_descr->flags && !pg_cache_descr->refcnt) {
+                rrdeng_destroy_pg_cache_descr(ctx, pg_cache_descr);
+                we_freed = 1;
+                /* success */
+                continue;
+            }
+            continue; /* spin */
+        }
+        if (unlikely(must_unlock)) {
+            assert(0 == old_users);
+
+            if (we_freed) {
+                /* success */
+                new_state = 0;
+            } else {
+                new_state = old_state | PG_CACHE_DESCR_DESTROY;
+                new_state &= ~PG_CACHE_DESCR_LOCKED;
+            }
+            ret_state = ulong_compare_and_swap(&descr->pg_cache_descr_state, old_state, new_state);
+            if (old_state == ret_state) {
+                /* unlocked */
+                return;
+            }
+            continue; /* spin */
+        }
+        if (!(old_state & PG_CACHE_DESCR_ALLOCATED)) {
+            /* don't do anything */
+            return;
+        }
+        if (old_state & PG_CACHE_DESCR_LOCKED) {
+            assert(0 == old_users);
+            continue; /* spin */
+        }
+        pg_cache_descr = descr->pg_cache_descr;
+        /* caller is the only page cache descriptor user */
+        if (0 == old_users) {
+            new_state = old_state | PG_CACHE_DESCR_LOCKED;
+            ret_state = ulong_compare_and_swap(&descr->pg_cache_descr_state, old_state, new_state);
+            if (old_state == ret_state) {
+                just_locked = 1;
+                /* retry */
+                continue;
+            }
+            continue; /* spin */
+        }
+        if (old_state & PG_CACHE_DESCR_DESTROY) {
+            /* don't do anything */
+            return;
+        }
+        /* plant PG_CACHE_DESCR_DESTROY so that other contexts eventually free the page cache descriptor */
+        new_state = old_state | PG_CACHE_DESCR_DESTROY;
+
+        ret_state = ulong_compare_and_swap(&descr->pg_cache_descr_state, old_state, new_state);
+        if (old_state == ret_state) {
+            /* success */
+            return;
+        }
+        /* spin */
+    }
+}
\ No newline at end of file
diff --git a/database/engine/rrdenglocking.h b/database/engine/rrdenglocking.h
new file mode 100644
index 000000000..127ddc90c
--- /dev/null
+++ b/database/engine/rrdenglocking.h
@@ -0,0 +1,17 @@
+// SPDX-License-Identifier: GPL-3.0-or-later
+
+#ifndef NETDATA_RRDENGLOCKING_H
+#define NETDATA_RRDENGLOCKING_H
+
+#include "rrdengine.h"
+
+/* Forward declarations */
+struct page_cache_descr;
+
+extern struct page_cache_descr *rrdeng_create_pg_cache_descr(struct rrdengine_instance *ctx);
+extern void rrdeng_destroy_pg_cache_descr(struct rrdengine_instance *ctx, struct page_cache_descr *pg_cache_descr);
+extern void rrdeng_page_descr_mutex_lock(struct rrdengine_instance *ctx, struct rrdeng_page_descr *descr);
+extern void rrdeng_page_descr_mutex_unlock(struct rrdengine_instance *ctx, struct rrdeng_page_descr *descr);
+extern void rrdeng_try_deallocate_pg_cache_descr(struct rrdengine_instance *ctx, struct rrdeng_page_descr *descr);
+
+#endif /* NETDATA_RRDENGLOCKING_H */
\ No newline at end of file