=================
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Writecache target
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=================
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The writecache target caches writes on persistent memory or on SSD. It
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doesn't cache reads because reads are supposed to be cached in page cache
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in normal RAM.
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When the device is constructed, the first sector should be zeroed or the
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first sector should contain valid superblock from previous invocation.
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Constructor parameters:
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1. type of the cache device - "p" or "s"
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- p - persistent memory
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- s - SSD
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2. the underlying device that will be cached
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3. the cache device
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4. block size (4096 is recommended; the maximum block size is the page
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size)
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5. the number of optional parameters (the parameters with an argument
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count as two)
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start_sector n (default: 0)
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offset from the start of cache device in 512-byte sectors
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high_watermark n (default: 50)
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start writeback when the number of used blocks reach this
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watermark
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low_watermark x (default: 45)
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stop writeback when the number of used blocks drops below
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this watermark
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writeback_jobs n (default: unlimited)
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limit the number of blocks that are in flight during
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writeback. Setting this value reduces writeback
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throughput, but it may improve latency of read requests
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autocommit_blocks n (default: 64 for pmem, 65536 for ssd)
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when the application writes this amount of blocks without
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issuing the FLUSH request, the blocks are automatically
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commited
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autocommit_time ms (default: 1000)
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autocommit time in milliseconds. The data is automatically
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commited if this time passes and no FLUSH request is
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received
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fua (by default on)
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applicable only to persistent memory - use the FUA flag
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when writing data from persistent memory back to the
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underlying device
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nofua
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applicable only to persistent memory - don't use the FUA
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flag when writing back data and send the FLUSH request
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afterwards
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- some underlying devices perform better with fua, some
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with nofua. The user should test it
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Status:
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1. error indicator - 0 if there was no error, otherwise error number
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2. the number of blocks
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3. the number of free blocks
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4. the number of blocks under writeback
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Messages:
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flush
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flush the cache device. The message returns successfully
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if the cache device was flushed without an error
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flush_on_suspend
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flush the cache device on next suspend. Use this message
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when you are going to remove the cache device. The proper
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sequence for removing the cache device is:
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1. send the "flush_on_suspend" message
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2. load an inactive table with a linear target that maps
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to the underlying device
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3. suspend the device
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4. ask for status and verify that there are no errors
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5. resume the device, so that it will use the linear
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target
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6. the cache device is now inactive and it can be deleted
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