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 24 start_sector n (default: 0) 25 offset from the start of cache device in 512-byte sectors 26 high_watermark n (default: 50) 27 start writeback when the number of used blocks reach this 28 watermark 29 low_watermark x (default: 45) 30 stop writeback when the number of used blocks drops below 31 this watermark 32 writeback_jobs n (default: unlimited) 33 limit the number of blocks that are in flight during 34 writeback. Setting this value reduces writeback 35 throughput, but it may improve latency of read requests 36 autocommit_blocks n (default: 64 for pmem, 65536 for ssd) 37 when the application writes this amount of blocks without 38 issuing the FLUSH request, the blocks are automatically 39 committed 40 autocommit_time ms (default: 1000) 41 autocommit time in milliseconds. The data is automatically 42 committed if this time passes and no FLUSH request is 43 received 44 fua (by default on) 45 applicable only to persistent memory - use the FUA flag 46 when writing data from persistent memory back to the 47 underlying device 48 nofua 49 applicable only to persistent memory - don't use the FUA 50 flag when writing back data and send the FLUSH request 51 afterwards 52 53 - some underlying devices perform better with fua, some 54 with nofua. The user should test it 55 cleaner 56 when this option is activated (either in the constructor 57 arguments or by a message), the cache will not promote 58 new writes (however, writes to already cached blocks are 59 promoted, to avoid data corruption due to misordered 60 writes) and it will gradually writeback any cached 61 data. The userspace can then monitor the cleaning 62 process with "dmsetup status". When the number of cached 63 blocks drops to zero, userspace can unload the 64 dm-writecache target and replace it with dm-linear or 65 other targets. 66 max_age n 67 specifies the maximum age of a block in milliseconds. If 68 a block is stored in the cache for too long, it will be 69 written to the underlying device and cleaned up. 70 metadata_only 71 only metadata is promoted to the cache. This option 72 improves performance for heavier REQ_META workloads. 73 pause_writeback n (default: 3000) 74 pause writeback if there was some write I/O redirected to 75 the origin volume in the last n milliseconds 76 77Status: 78 791. error indicator - 0 if there was no error, otherwise error number 802. the number of blocks 813. the number of free blocks 824. the number of blocks under writeback 835. the number of read blocks 846. the number of read blocks that hit the cache 857. the number of write blocks 868. the number of write blocks that hit uncommitted block 879. the number of write blocks that hit committed block 8810. the number of write blocks that bypass the cache 8911. the number of write blocks that are allocated in the cache 9012. the number of write requests that are blocked on the freelist 9113. the number of flush requests 9214. the number of discarded blocks 93 94Messages: 95 flush 96 Flush the cache device. The message returns successfully 97 if the cache device was flushed without an error 98 flush_on_suspend 99 Flush the cache device on next suspend. Use this message 100 when you are going to remove the cache device. The proper 101 sequence for removing the cache device is: 102 103 1. send the "flush_on_suspend" message 104 2. load an inactive table with a linear target that maps 105 to the underlying device 106 3. suspend the device 107 4. ask for status and verify that there are no errors 108 5. resume the device, so that it will use the linear 109 target 110 6. the cache device is now inactive and it can be deleted 111 cleaner 112 See above "cleaner" constructor documentation. 113 clear_stats 114 Clear the statistics that are reported on the status line 115