1. 17 Jun, 2015 2 commits
    • Mike Snitzer's avatar
      dm cache: switch the "default" cache replacement policy from mq to smq · bccab6a0
      Mike Snitzer authored
      The Stochastic multiqueue (SMQ) policy (vs MQ) offers the promise of
      less memory utilization, improved performance and increased adaptability
      in the face of changing workloads.  SMQ also does not have any
      cumbersome tuning knobs.
      
      Users may switch from "mq" to "smq" simply by appropriately reloading a
      DM table that is using the cache target.  Doing so will cause all of the
      mq policy's hints to be dropped.  Also, performance of the cache may
      degrade slightly until smq recalculates the origin device's hotspots
      that should be cached.
      
      In the future the "mq" policy will just silently make use of "smq" and
      the mq code will be removed.
      Signed-off-by: default avatarMike Snitzer <snitzer@redhat.com>
      Acked-by: default avatarJoe Thornber <ejt@redhat.com>
      bccab6a0
    • Joe Thornber's avatar
      dm space map metadata: fix occasional leak of a metadata block on resize · 6096d91a
      Joe Thornber authored
      The metadata space map has a simplified 'bootstrap' mode that is
      operational when extending the space maps.  Whilst in this mode it's
      possible for some refcount decrement operations to become queued (eg, as
      a result of shadowing one of the bitmap indexes).  These decrements were
      not being applied when switching out of bootstrap mode.
      
      The effect of this bug was the leaking of a 4k metadata block.  This is
      detected by the latest version of thin_check as a non fatal error.
      Signed-off-by: default avatarJoe Thornber <ejt@redhat.com>
      Signed-off-by: default avatarMike Snitzer <snitzer@redhat.com>
      Cc: stable@vger.kernel.org
      6096d91a
  2. 11 Jun, 2015 12 commits
    • Joe Thornber's avatar
      dm thin metadata: fix a race when entering fail mode · b1f11aff
      Joe Thornber authored
      In dm_thin_find_block() the ->fail_io flag was checked outside the
      metadata device's root_lock, causing dm_thin_find_block() to race with
      the setting of this flag.
      Signed-off-by: default avatarJoe Thornber <ejt@redhat.com>
      Signed-off-by: default avatarMike Snitzer <snitzer@redhat.com>
      b1f11aff
    • Mike Snitzer's avatar
      dm thin: fail messages with EOPNOTSUPP when pool cannot handle messages · fd467696
      Mike Snitzer authored
      Use EOPNOTSUPP, rather than EINVAL, error code when user attempts to
      send the pool a message.  Otherwise usespace is led to believe the
      message failed due to invalid argument.
      Reported-by: default avatarZdenek Kabelac <zkabelac@redhat.com>
      Signed-off-by: default avatarMike Snitzer <snitzer@redhat.com>
      fd467696
    • Joe Thornber's avatar
      dm thin: range discard support · 34fbcf62
      Joe Thornber authored
      Previously REQ_DISCARD bios have been split into block sized chunks
      before submission to the thin target.  There are a couple of issues with
      this:
      
       - If the block size is small, a large discard request can
         get broken up into a great many bios which is both slow and causes
         a lot of memory pressure.
      
       - The thin pool block size and the discard granularity for the
         underlying data device need to be compatible if we want to passdown
         the discard.
      
      This patch relaxes the block size granularity for thin devices.  It
      makes use of the recent range locking added to the bio_prison to
      quiesce a whole range of thin blocks before unmapping them.  Once a
      thin range has been unmapped the discard can then be passed down to
      the data device for those sub ranges where the data blocks are no
      longer used (ie. they weren't shared in the first place).
      
      This patch also doesn't make any apologies about open-coding portions
      of block core as a means to supporting async discard completions in the
      near-term -- if/when late bio splitting lands it'll all get cleaned up.
      Signed-off-by: default avatarJoe Thornber <ejt@redhat.com>
      Signed-off-by: default avatarMike Snitzer <snitzer@redhat.com>
      34fbcf62
    • Joe Thornber's avatar
      dm thin metadata: add dm_thin_remove_range() · 6550f075
      Joe Thornber authored
      Removes a range of blocks from the btree.
      Signed-off-by: default avatarJoe Thornber <ejt@redhat.com>
      Signed-off-by: default avatarMike Snitzer <snitzer@redhat.com>
      6550f075
    • Joe Thornber's avatar
      dm thin metadata: add dm_thin_find_mapped_range() · a5d895a9
      Joe Thornber authored
      Retrieve the next run of contiguously mapped blocks.  Useful for working
      out where to break up IO.
      Signed-off-by: default avatarJoe Thornber <ejt@redhat.com>
      Signed-off-by: default avatarMike Snitzer <snitzer@redhat.com>
      a5d895a9
    • Joe Thornber's avatar
      dm btree: add dm_btree_remove_leaves() · 4ec331c3
      Joe Thornber authored
      Removes a range of leaf values from the tree.
      Signed-off-by: default avatarJoe Thornber <ejt@redhat.com>
      Signed-off-by: default avatarMike Snitzer <snitzer@redhat.com>
      4ec331c3
    • Pekka Enberg's avatar
      dm stats: Use kvfree() in dm_kvfree() · 0f24b79b
      Pekka Enberg authored
      Use kvfree() instead of open-coding it.
      Signed-off-by: default avatarPekka Enberg <penberg@kernel.org>
      Signed-off-by: default avatarMike Snitzer <snitzer@redhat.com>
      0f24b79b
    • Joe Thornber's avatar
      dm cache: age and write back cache entries even without active IO · fba10109
      Joe Thornber authored
      The policy tick() method is normally called from interrupt context.
      Both the mq and smq policies do some bottom half work for the tick
      method in their map functions.  However if no IO is going through the
      cache, then that bottom half work doesn't occur.  With these policies
      this means recently hit entries do not age and do not get written
      back as early as we'd like.
      
      Fix this by introducing a new 'can_block' parameter to the tick()
      method.  When this is set the bottom half work occurs immediately.
      'can_block' is set when the tick method is called every second by the
      core target (not in interrupt context).
      Signed-off-by: default avatarJoe Thornber <ejt@redhat.com>
      Signed-off-by: default avatarMike Snitzer <snitzer@redhat.com>
      fba10109
    • Mike Snitzer's avatar
      dm cache: prefix all DMERR and DMINFO messages with cache device name · b61d9509
      Mike Snitzer authored
      Having the DM device name associated with the ERR or INFO message is
      very helpful.
      Signed-off-by: default avatarMike Snitzer <snitzer@redhat.com>
      b61d9509
    • Joe Thornber's avatar
      dm cache: add fail io mode and needs_check flag · 028ae9f7
      Joe Thornber authored
      If a cache metadata operation fails (e.g. transaction commit) the
      cache's metadata device will abort the current transaction, set a new
      needs_check flag, and the cache will transition to "read-only" mode.  If
      aborting the transaction or setting the needs_check flag fails the cache
      will transition to "fail-io" mode.
      
      Once needs_check is set the cache device will not be allowed to
      activate.  Activation requires write access to metadata.  Future work is
      needed to add proper support for running the cache in read-only mode.
      
      Once in fail-io mode the cache will report a status of "Fail".
      
      Also, add commit() wrapper that will disallow commits if in read_only or
      fail mode.
      Signed-off-by: default avatarJoe Thornber <ejt@redhat.com>
      Signed-off-by: default avatarMike Snitzer <snitzer@redhat.com>
      028ae9f7
    • Joe Thornber's avatar
      dm cache: wake the worker thread every time we free a migration object · 88bf5184
      Joe Thornber authored
      When the cache is idle, writeback work was only being issued every
      second.  With this change outstanding writebacks are streamed
      constantly.  This offers a writeback performance improvement.
      Signed-off-by: default avatarJoe Thornber <ejt@redhat.com>
      Signed-off-by: default avatarMike Snitzer <snitzer@redhat.com>
      88bf5184
    • Joe Thornber's avatar
      dm cache: add stochastic-multi-queue (smq) policy · 66a63635
      Joe Thornber authored
      The stochastic-multi-queue (smq) policy addresses some of the problems
      with the current multiqueue (mq) policy.
      
      Memory usage
      ------------
      
      The mq policy uses a lot of memory; 88 bytes per cache block on a 64
      bit machine.
      
      SMQ uses 28bit indexes to implement it's data structures rather than
      pointers.  It avoids storing an explicit hit count for each block.  It
      has a 'hotspot' queue rather than a pre cache which uses a quarter of
      the entries (each hotspot block covers a larger area than a single
      cache block).
      
      All these mean smq uses ~25bytes per cache block.  Still a lot of
      memory, but a substantial improvement nontheless.
      
      Level balancing
      ---------------
      
      MQ places entries in different levels of the multiqueue structures
      based on their hit count (~ln(hit count)).  This means the bottom
      levels generally have the most entries, and the top ones have very
      few.  Having unbalanced levels like this reduces the efficacy of the
      multiqueue.
      
      SMQ does not maintain a hit count, instead it swaps hit entries with
      the least recently used entry from the level above.  The over all
      ordering being a side effect of this stochastic process.  With this
      scheme we can decide how many entries occupy each multiqueue level,
      resulting in better promotion/demotion decisions.
      
      Adaptability
      ------------
      
      The MQ policy maintains a hit count for each cache block.  For a
      different block to get promoted to the cache it's hit count has to
      exceed the lowest currently in the cache.  This means it can take a
      long time for the cache to adapt between varying IO patterns.
      Periodically degrading the hit counts could help with this, but I
      haven't found a nice general solution.
      
      SMQ doesn't maintain hit counts, so a lot of this problem just goes
      away.  In addition it tracks performance of the hotspot queue, which
      is used to decide which blocks to promote.  If the hotspot queue is
      performing badly then it starts moving entries more quickly between
      levels.  This lets it adapt to new IO patterns very quickly.
      
      Performance
      -----------
      
      In my tests SMQ shows substantially better performance than MQ.  Once
      this matures a bit more I'm sure it'll become the default policy.
      Signed-off-by: default avatarJoe Thornber <ejt@redhat.com>
      Signed-off-by: default avatarMike Snitzer <snitzer@redhat.com>
      66a63635
  3. 29 May, 2015 25 commits
  4. 27 May, 2015 1 commit