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(de)fragmentation doc #30

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cmurf opened this issue Feb 7, 2021 · 0 comments
Open

(de)fragmentation doc #30

cmurf opened this issue Feb 7, 2021 · 0 comments

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@cmurf
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cmurf commented Feb 7, 2021

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
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