(de)fragmentation doc

[cmurf]

A doc that helps application developers manage fragmentation:

• estimating fragmentation
  • limitations of FIEMAP (filefrag)
  • effect of compression on real fragmentation vs as reported by FIEMAP
• avoidance strategies
  • nodatacow, pros/cons
  • fallocate, pros/cons
  • create a subvolume in lieu of an enclosing directory for the files? helps avoid inadvertent snapshots from above
• submitting files for periodic defragmentation
  • BTRFS_IOC_DEFRAG vs BTRFS_IOC_DEFRAG_RANGE (should libbtrfsutil expose a preferred usage?)
  • autodefrag by xattr pros/cons (future feature)
  • effect of defrag on reflinked files; e.g. defrag is not snapshot aware
  • estimating the cost/benefit of defragmenting (latency?)
• misc/appendix
  • effect of nossd, ssd, ssd_spread on fragmentation and defragmentation
  • kinds of fragmentation: free space, metadata, data