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 commited

    autocommit_time ms (default: 1000)

    autocommit time in milliseconds. The data is automatically commited 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


    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

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

flush the cache device. The message returns successfully if the cache device was flushed without an error

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