Linux kernel mirror (for testing)
git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git
tjh.dev
kernel
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linux
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