[PATCH 0/2] btrfs: defrag: better lone extent handling

[Qu Wenruo <wqu@suse.com>]

When a lone file extent (a file extent without any adjacent non-hole
file extent) is involved, it would haven no chance to be touched by
defrag.

This would mean that, if we have some lone extents with very low
utilization ratio, or defragging it can free up a lot of space, defrag
would not touch them no matter what.

This is not ideal for some situations, e.g.:

  # mkfs.btrfs -f $dev
  # mount $dev $mnt
  # xfs_io -f -c "pwrite 0 128M" $mnt/foobar
  # sync
  # truncate -s 4k $mnt/foobar
  # btrfs filesystem defrag $mnt/foobar
  # sync

In above case, if defrag touches the 4k extent, it would free up the
whole 128M extent, which should be a good win.

This patchset would address the problem by introducing a special
entrance for lone file extents.
Lone file extents meeting either ratio or wasted bytes threshold would
be considered as a defrag target, allowing end uesrs to address above
situation.

This change requires progs support (or direct ioctl() calling), by
default they would be disabled.

And my personal recommendation for the ratio would be (4069 / 65536)
(aka, 1/16 of the on-disk file extent size), and 16MiB (if freeing up
the extent can free more than 16MiB, excluding the lone extent size).

Qu Wenruo (2):
  btrfs: defrag: add under utilized extent to defrag target list
  btrfs: defrag: allow fine-tuning on lone extent defrag behavior

 fs/btrfs/defrag.c          | 68 +++++++++++++++++++++++++++++++++-----
 fs/btrfs/ioctl.c           |  9 +++++
 include/uapi/linux/btrfs.h | 28 +++++++++++++---
 3 files changed, 92 insertions(+), 13 deletions(-)

-- 
2.43.0