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=================
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.
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
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.
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