summaryrefslogtreecommitdiffstats
path: root/doc/dev/osd_internals/wbthrottle.rst
diff options
context:
space:
mode:
Diffstat (limited to 'doc/dev/osd_internals/wbthrottle.rst')
-rw-r--r--doc/dev/osd_internals/wbthrottle.rst28
1 files changed, 28 insertions, 0 deletions
diff --git a/doc/dev/osd_internals/wbthrottle.rst b/doc/dev/osd_internals/wbthrottle.rst
new file mode 100644
index 000000000..9b67efbb6
--- /dev/null
+++ b/doc/dev/osd_internals/wbthrottle.rst
@@ -0,0 +1,28 @@
+==================
+Writeback Throttle
+==================
+
+Previously, the filestore had a problem when handling large numbers of
+small ios. We throttle dirty data implicitly via the journal, but
+a large number of inodes can be dirtied without filling the journal
+resulting in a very long sync time when the sync finally does happen.
+The flusher was not an adequate solution to this problem since it
+forced writeback of small writes too eagerly killing performance.
+
+WBThrottle tracks unflushed io per hobject_t and ::fsyncs in lru
+order once the start_flusher threshold is exceeded for any of
+dirty bytes, dirty ios, or dirty inodes. While any of these exceed
+the hard_limit, we block on throttle() in _do_op.
+
+See src/os/WBThrottle.h, src/osd/WBThrottle.cc
+
+To track the open FDs through the writeback process, there is now an
+fdcache to cache open fds. lfn_open now returns a cached FDRef which
+implicitly closes the fd once all references have expired.
+
+Filestore syncs have a sideeffect of flushing all outstanding objects
+in the wbthrottle.
+
+lfn_unlink clears the cached FDRef and wbthrottle entries for the
+unlinked object when the last link is removed and asserts that all
+outstanding FDRefs for that object are dead.