ce2521b407
flushoncommit or not-flushoncommit isn't really a bees matter--it's a sysadmin's tradeoff between reliability and performance. bees does not affect that tradeoff because all dedupe src extents are flushed, so bees introduces no *new* data loss risks in the noflushoncommit case--i.e. any data that you could lose while running bees, you'd also lose when not running bees. Note that the converse is not true: bees might trigger flushing on data that would not normally have been flushed with noflushoncommit, and improve data integrity after a crash as a side-effect of dedupe operations. The risks of noflushoncommit might be reduced by running bees. I don't have evidence based on experimental data to support that conclusion, so I'll just leave this possibility as a rumor in a commit log message. lvmcache can be moved from the "bad" list to the "good" list now. bcache remains in the "bad" list due to some non-data-losing failures that only seem to happen with bcache. Add a note about CPUs with strange endianness or page sizes, as nobody seems to have tried those. Remove "at great cost" from the btrfs send workaround. The cost is the cost, there is no need to editorialize. Signed-off-by: Zygo Blaxell <bees@furryterror.org>
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Good Btrfs Feature Interactions
bees has been tested in combination with the following:
- btrfs compression (zlib, lzo, zstd), mixtures of compressed and uncompressed extents
- PREALLOC extents (unconditionally replaced with holes)
- HOLE extents and btrfs no-holes feature
- Other deduplicators, reflink copies (though bees may decide to redo their work)
- btrfs snapshots and non-snapshot subvols (RW and RO)
- Concurrent file modification (e.g. PostgreSQL and sqlite databases, build daemons)
- all btrfs RAID profiles
- IO errors during dedupe (read errors will throw exceptions, bees will catch them and skip over the affected extent)
- Filesystems mounted with the flushoncommit option (lots of harmless kernel log warnings on 4.15 and later)
- Filesystems mounted without the flushoncommit option
- 4K filesystem data block size / clone alignment
- 64-bit and 32-bit LE host CPUs (amd64, x86, arm)
- Huge files (>1TB--although Btrfs performance on such files isn't great in general)
- filesystems up to 30T+ bytes, 100M+ files
- btrfs receive
- btrfs nodatacow/nodatasum inode attribute or mount option (bees skips all nodatasum files)
- open(O_DIRECT) (seems to work as well--or as poorly--with bees as with any other btrfs feature)
- lvmcache: no problems observed in testing with recent kernels or reported by users in the last year.
Bad Btrfs Feature Interactions
bees has been tested in combination with the following, and various problems are known:
-
bcache: no data-losing problems observed in testing with recent kernels or reported by users in the last year. Some issues observed with bcache interacting badly with some SSD models' firmware, but so far this only causes temporary loss of service, not filesystem damage. This behavior does not seem to be specific to bees (ordinary filesystem tests with rsync and snapshots will reproduce it), but it does prevent any significant testing of bees on bcache.
-
btrfs send: there are bugs in
btrfs sendthat can be triggered by bees. The--workaround-btrfs-sendoption works around this issue by preventing bees from modifying read-only snapshots. -
btrfs qgroups: very slow, sometimes hangs...and it's even worse when bees is running.
-
btrfs autodefrag mount option: hangs and high CPU usage problems reported by users. bees cannot distinguish autodefrag activity from normal filesystem activity and will likely try to undo the autodefrag if duplicate copies of the defragmented data exist.
Untested Btrfs Feature Interactions
bees has not been tested with the following, and undesirable interactions may occur:
- Non-4K filesystem data block size (should work if recompiled)
- Non-equal hash (SUM) and filesystem data block (CLONE) sizes (need to fix that eventually)
- btrfs seed filesystems (does anyone even use those?)
- btrfs out-of-tree kernel patches (e.g. in-kernel dedupe or encryption)
- btrfs-convert from ext2/3/4 (never tested, might run out of space or ignore significant portions of the filesystem due to sanity checks)
- btrfs mixed block groups (don't know a reason why it would not work, but never tested)
- flashcache: an out-of-tree cache-HDD-on-SSD block layer helper.
- Host CPUs with exotic page sizes, alignment requirements, or endianness (ppc, alpha, sparc, strongarm, s390, mips, m68k...)