defrag-v2.txt

Update to defrag ioctl, v2
--------------------------

There are some feature and functionality requests or things to fix.

Compression:

- level - add ability to specify level under which the defragmentation should
  happen; this can be different completely what is the current compression for
  the file or for the mount, so defrag overrides everything ("user knows what
  to do"), we can possibly add a shortcut to only raise the level from the
  current so it's not necessary to know which one it is (most likely zstd)

- none - force no compression on the range, ie. if the file is compressed then
  rewrite all extents uncompressed (obviously breaking sharing), otherwise
  apply same logic for extent filtering

- sticky - also set the compression method/level (if implemented) for the
  file, either at the beginning of defrag so it's clear when it starts, or at
  the end (until then the current setting would applie, maybe this is not
  wanted)

Extent sharing:

- add flags for extent sharing or breaking request
  - we don't have snapshot aware defrag anymore but at least we can provide
    the interface and then make it work
  - extent breaking will be the default and the only one working

Defrag metadata:

- running defrag ioctl on a directory does actually defragment metadata of the
  containing subvolume, this can be useful in case the directory or subvolume
  metadata become fragmented
  - we can make it defrag only the blocks related to the directory, ie keys
    with DIR_ENTRY with the dir inode, subset of the whole subvolume
  - we can have both, either directory or the whole subvolume

Extent slack, bookend extents:

- portions of extents can be completely unreachable due to bookend extents
  mechanism, this still occupies space and is otherwise skipped
  - add a filter that selects extents that have a given amount of data
    invisible and defrag only the live range, drop the rest ("make it compact")


Notes:

- space in the ioctl structure is getting tight, something can be pushed to
  flags only but for anything with range will probably need new space

- a new ioctl number is possible but only as last resort