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diff --git a/Documentation/admin-guide/device-mapper/writecache.rst b/Documentation/admin-guide/device-mapper/writecache.rst new file mode 100644 index 0000000000..60c16b7fd5 --- /dev/null +++ b/Documentation/admin-guide/device-mapper/writecache.rst @@ -0,0 +1,114 @@ +================= +Writecache target +================= + +The writecache target caches writes on persistent memory or on SSD. It +doesn't cache reads because reads are supposed to be cached in page cache +in normal RAM. + +When the device is constructed, the first sector should be zeroed or the +first sector should contain valid superblock from previous invocation. + +Constructor parameters: + +1. type of the cache device - "p" or "s" + - p - persistent memory + - s - SSD +2. the underlying device that will be cached +3. the cache device +4. block size (4096 is recommended; the maximum block size is the page + size) +5. the number of optional parameters (the parameters with an argument + count as two) + + start_sector n (default: 0) + offset from the start of cache device in 512-byte sectors + high_watermark n (default: 50) + start writeback when the number of used blocks reach this + watermark + low_watermark x (default: 45) + stop writeback when the number of used blocks drops below + this watermark + writeback_jobs n (default: unlimited) + limit the number of blocks that are in flight during + writeback. Setting this value reduces writeback + throughput, but it may improve latency of read requests + autocommit_blocks n (default: 64 for pmem, 65536 for ssd) + when the application writes this amount of blocks without + issuing the FLUSH request, the blocks are automatically + committed + autocommit_time ms (default: 1000) + autocommit time in milliseconds. The data is automatically + committed if this time passes and no FLUSH request is + received + fua (by default on) + applicable only to persistent memory - use the FUA flag + when writing data from persistent memory back to the + underlying device + nofua + applicable only to persistent memory - don't use the FUA + flag when writing back data and send the FLUSH request + afterwards + + - some underlying devices perform better with fua, some + with nofua. The user should test it + cleaner + when this option is activated (either in the constructor + arguments or by a message), the cache will not promote + new writes (however, writes to already cached blocks are + promoted, to avoid data corruption due to misordered + writes) and it will gradually writeback any cached + data. The userspace can then monitor the cleaning + process with "dmsetup status". When the number of cached + blocks drops to zero, userspace can unload the + dm-writecache target and replace it with dm-linear or + other targets. + max_age n + specifies the maximum age of a block in milliseconds. If + a block is stored in the cache for too long, it will be + written to the underlying device and cleaned up. + metadata_only + only metadata is promoted to the cache. This option + improves performance for heavier REQ_META workloads. + pause_writeback n (default: 3000) + pause writeback if there was some write I/O redirected to + the origin volume in the last n milliseconds + +Status: + +1. error indicator - 0 if there was no error, otherwise error number +2. the number of blocks +3. the number of free blocks +4. the number of blocks under writeback +5. the number of read blocks +6. the number of read blocks that hit the cache +7. the number of write blocks +8. the number of write blocks that hit uncommitted block +9. the number of write blocks that hit committed block +10. the number of write blocks that bypass the cache +11. the number of write blocks that are allocated in the cache +12. the number of write requests that are blocked on the freelist +13. the number of flush requests +14. the number of discarded blocks + +Messages: + flush + Flush the cache device. The message returns successfully + if the cache device was flushed without an error + flush_on_suspend + Flush the cache device on next suspend. Use this message + when you are going to remove the cache device. The proper + sequence for removing the cache device is: + + 1. send the "flush_on_suspend" message + 2. load an inactive table with a linear target that maps + to the underlying device + 3. suspend the device + 4. ask for status and verify that there are no errors + 5. resume the device, so that it will use the linear + target + 6. the cache device is now inactive and it can be deleted + cleaner + See above "cleaner" constructor documentation. + clear_stats + Clear the statistics that are reported on the status line |