1. 29 5月, 2018 3 次提交
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      Btrfs: implement the free space B-tree · a5ed9182
      Omar Sandoval 提交于
      The free space cache has turned out to be a scalability bottleneck on
      large, busy filesystems. When the cache for a lot of block groups needs
      to be written out, we can get extremely long commit times; if this
      happens in the critical section, things are especially bad because we
      block new transactions from happening.
      
      The main problem with the free space cache is that it has to be written
      out in its entirety and is managed in an ad hoc fashion. Using a B-tree
      to store free space fixes this: updates can be done as needed and we get
      all of the benefits of using a B-tree: checksumming, RAID handling,
      well-understood behavior.
      
      With the free space tree, we get commit times that are about the same as
      the no cache case with load times slower than the free space cache case
      but still much faster than the no cache case. Free space is represented
      with extents until it becomes more space-efficient to use bitmaps,
      giving us similar space overhead to the free space cache.
      
      The operations on the free space tree are: adding and removing free
      space, handling the creation and deletion of block groups, and loading
      the free space for a block group. We can also create the free space tree
      by walking the extent tree and clear the free space tree.
      Signed-off-by: NOmar Sandoval <osandov@fb.com>
      Signed-off-by: NChris Mason <clm@fb.com>
      a5ed9182