git: HAMMER VFS - Remove B-Tree allocation hints, add double_buffer option.

Matthew Dillon dillon at
Tue Jan 11 19:51:43 PST 2011

commit b4f86ea3cc005dec41539fde163eeb5606e81cb8
Author: Matthew Dillon <dillon at>
Date:   Tue Jan 11 19:43:23 2011 -0800

    HAMMER VFS - Remove B-Tree allocation hints, add double_buffer option.
    * Remove the allocation hints when allocating b-tree nodes and
      remove over-full test in the blockmap allocator for b-tree and
      meta-data elements.
      The hinting and leaving some space unused in the big-blocks did
      not improve performance.  Write performance is actually slightly
      better when new allocations are made linearly.
    * Either way we have to depend on the reblocker to reorganize the
    * Add a sysctl vfs.hammer.double_buffer, defaulting to off.  This
      is currently used for debugging and testing live-dedup.
      Normally only small-data blocks are run through the device vnode's
      buffer cache (allowing us to consolidate many small data blocks
      within the device vnode's buffer cache), and large data blocks are
      read directly into the file vnode's buffer.
      Turning on double_buffer cases ALL file data to run through the
      device vnode's buffer cache, resulting in double data caching
      which is not normally useful, so leave this off for now.  It will
      not improve performance.

Summary of changes:
 sys/vfs/hammer/hammer.h          |    1 +
 sys/vfs/hammer/hammer_blockmap.c |    2 ++
 sys/vfs/hammer/hammer_btree.c    |   10 ++++------
 sys/vfs/hammer/hammer_reblock.c  |    2 +-
 sys/vfs/hammer/hammer_vfsops.c   |    3 +++
 sys/vfs/hammer/hammer_vnops.c    |   20 +++++++++++++++-----
 6 files changed, 26 insertions(+), 12 deletions(-)

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