writecache.rst (4347B)
1================= 2Writecache target 3================= 4 5The writecache target caches writes on persistent memory or on SSD. It 6doesn't cache reads because reads are supposed to be cached in page cache 7in normal RAM. 8 9When the device is constructed, the first sector should be zeroed or the 10first sector should contain valid superblock from previous invocation. 11 12Constructor parameters: 13 141. type of the cache device - "p" or "s" 15 - p - persistent memory 16 - s - SSD 172. the underlying device that will be cached 183. the cache device 194. block size (4096 is recommended; the maximum block size is the page 20 size) 215. the number of optional parameters (the parameters with an argument 22 count as two) 23 start_sector n (default: 0) 24 offset from the start of cache device in 512-byte sectors 25 high_watermark n (default: 50) 26 start writeback when the number of used blocks reach this 27 watermark 28 low_watermark x (default: 45) 29 stop writeback when the number of used blocks drops below 30 this watermark 31 writeback_jobs n (default: unlimited) 32 limit the number of blocks that are in flight during 33 writeback. Setting this value reduces writeback 34 throughput, but it may improve latency of read requests 35 autocommit_blocks n (default: 64 for pmem, 65536 for ssd) 36 when the application writes this amount of blocks without 37 issuing the FLUSH request, the blocks are automatically 38 committed 39 autocommit_time ms (default: 1000) 40 autocommit time in milliseconds. The data is automatically 41 committed if this time passes and no FLUSH request is 42 received 43 fua (by default on) 44 applicable only to persistent memory - use the FUA flag 45 when writing data from persistent memory back to the 46 underlying device 47 nofua 48 applicable only to persistent memory - don't use the FUA 49 flag when writing back data and send the FLUSH request 50 afterwards 51 52 - some underlying devices perform better with fua, some 53 with nofua. The user should test it 54 cleaner 55 when this option is activated (either in the constructor 56 arguments or by a message), the cache will not promote 57 new writes (however, writes to already cached blocks are 58 promoted, to avoid data corruption due to misordered 59 writes) and it will gradually writeback any cached 60 data. The userspace can then monitor the cleaning 61 process with "dmsetup status". When the number of cached 62 blocks drops to zero, userspace can unload the 63 dm-writecache target and replace it with dm-linear or 64 other targets. 65 max_age n 66 specifies the maximum age of a block in milliseconds. If 67 a block is stored in the cache for too long, it will be 68 written to the underlying device and cleaned up. 69 metadata_only 70 only metadata is promoted to the cache. This option 71 improves performance for heavier REQ_META workloads. 72 pause_writeback n (default: 3000) 73 pause writeback if there was some write I/O redirected to 74 the origin volume in the last n milliseconds 75 76Status: 771. error indicator - 0 if there was no error, otherwise error number 782. the number of blocks 793. the number of free blocks 804. the number of blocks under writeback 815. the number of read requests 826. the number of read requests that hit the cache 837. the number of write requests 848. the number of write requests that hit uncommitted block 859. the number of write requests that hit committed block 8610. the number of write requests that bypass the cache 8711. the number of write requests that are allocated in the cache 8812. the number of write requests that are blocked on the freelist 8913. the number of flush requests 9014. the number of discard requests 91 92Messages: 93 flush 94 Flush the cache device. The message returns successfully 95 if the cache device was flushed without an error 96 flush_on_suspend 97 Flush the cache device on next suspend. Use this message 98 when you are going to remove the cache device. The proper 99 sequence for removing the cache device is: 100 101 1. send the "flush_on_suspend" message 102 2. load an inactive table with a linear target that maps 103 to the underlying device 104 3. suspend the device 105 4. ask for status and verify that there are no errors 106 5. resume the device, so that it will use the linear 107 target 108 6. the cache device is now inactive and it can be deleted 109 cleaner 110 See above "cleaner" constructor documentation. 111 clear_stats 112 Clear the statistics that are reported on the status line