[PATCH v4 0/2] btrfs: defrag: better handling for extents which can not be merged with adjacent ones

[Qu Wenruo <wqu@suse.com>]

[CHANGELOG]
v4:
- Remove the "*_LONE_*" mentioning in uapi

- Add extra initialization for the two new members
  This is to avoid fuzz tests to set those two new members without
  setting the coresponding flags.

v3:
- Use div_u64() for percentage usage_ratio calculation
  v1 uses "/ 65536" which compiler just optimized to right shift, and
  avoided u64 division.

v2:
- Remove the "lone" naming
  Now the two new members would be named "usage_ratio" and
  "wasted_bytes".
  
- Make "usage_ratio" to be in range [0, 100]
  This should be much easier to understand.

When a file extent which can not be merged with any adjacent ones (e.g.
created by truncating a large file extent) is involved, it would haven no
chance to be touched by defrag.

This would mean that, if we have some truncated 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 such file extents.
Those file extents meeting either usage 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 5%, and 16MiB.


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

 fs/btrfs/defrag.c          | 46 +++++++++++++++++++++++++++++++++++---
 fs/btrfs/ioctl.c           | 13 +++++++++++
 include/uapi/linux/btrfs.h | 40 ++++++++++++++++++++++++++++-----
 3 files changed, 91 insertions(+), 8 deletions(-)

-- 
2.44.0