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The main gains here are: * Move extent tree searches into BeesScanModeExtent so that they are not slowed down by the BeesCrawl code, which was designed for the much more specialized metadata in subvol trees. * Enable short extent skipping now that BeesCrawl is out of the way. * Stop enumerating btrfs subvols when in extent scan mode. All this gets rid of >99% of unnecessary extent tree searches. Incremental extent scan cycles now finish in milliseconds instead of minutes. BeesCrawl was never designed to cope with the structure and content of the extent tree. It would waste thousands of tree-search ioctl calls reading and ignoring metadata items. Performance was particularly bad when a binary search was involved, as any binary search probe that landed in a metadata block group would read and discard all the metadata items in the block group, sequentially, repeated for each level of the binary search. This was blocking implementation of short extent skipping optimization for large extent size tiers, because the skips were using thousands of tree searches to skip over only a few hundred extent items. Extent scan also had to read every extent item twice to do the transid filtering, because BeesCrawl's interface discarded the relevant information when it converted a `BtrfsTreeItem` into a `BeesFileRange`. The cost of this extra fetch was negligible, but it could have been zero. Fix this by: * Copy the equivalent of `fetch_extents` from BeesCrawl into `BeesScanModeExtent`, then give each of the extent scan crawlers its own `BtrfsDataExtentTreeFetcher` instance. This enables extent tree searches to avoid pure (non-mixed) metadata block groups. `BeesCrawl` is now used only for its interface to `BeesRoots` for saving state in `beescrawl.dat`, and never to determine the next extent tree item. * Move subvol-specific parts of `BeesRoots` into a new class `BeesScanModeSubvol` so that `BtrfsScanModeExtent` doesn't have to enable or support them. In particular, `bees -m4` no longer enumerates all of the _subvol_ crawlers. `BeesRoots` is still used to save and load crawl state. * Move several members from `BtrfsScanModeExtent` into a per-crawler state object `SizeTier` to eliminate the need for some locks and to maintain separate cache state for `BtrfsDataExtentTreeFetcher`. * Reuse the `BtrfsTreeItem` to get the generation field for the transid range filter. * Avoid a few corner cases when handling errors, where extent scan might drop an extent without scanning it, or fail to advance to the next extent. * Enable the extent-skipping algorithm for large size tiers, now that `BeesCrawl::fetch_extents` is no longer slowing it down. * Add a debug stream interface which developers can easily turn on when needed to inspect the decisions that extent scan is making. * Track metrics that are more useful, particularly searches per extent scanned, and fraction of extents that are skipped. Signed-off-by: Zygo Blaxell <bees@furryterror.org>
BEES
Best-Effort Extent-Same, a btrfs deduplication agent.
About bees
bees is a block-oriented userspace deduplication agent designed to scale up to large btrfs filesystems. It is an offline dedupe combined with an incremental data scan capability to minimize time data spends on disk from write to dedupe.
Strengths
- Space-efficient hash table - can use as little as 1 GB hash table per 10 TB unique data (0.1GB/TB)
- Daemon mode - incrementally dedupes new data as it appears
- Largest extents first - recover more free space during fixed maintenance windows
- Works with btrfs compression - dedupe any combination of compressed and uncompressed files
- Whole-filesystem dedupe - scans data only once, even with snapshots and reflinks
- Persistent hash table for rapid restart after shutdown
- Constant hash table size - no increased RAM usage if data set becomes larger
- Works on live data - no scheduled downtime required
- Automatic self-throttling - reduces system load
- btrfs support - recovers more free space from btrfs than naive dedupers
Weaknesses
- Whole-filesystem dedupe - has no include/exclude filters, does not accept file lists
- Requires root privilege (
CAP_SYS_ADMINplus the usual filesystem read/modify caps) - First run may increase metadata space usage if many snapshots exist
- Constant hash table size - no decreased RAM usage if data set becomes smaller
- btrfs only
Installation and Usage
Recommended Reading
- bees Gotchas
- btrfs kernel bugs - especially DATA CORRUPTION WARNING for old kernels
- bees vs. other btrfs features
- What to do when something goes wrong
More Information
Bug Reports and Contributions
Email bug reports and patches to Zygo Blaxell bees@furryterror.org.
You can also use Github:
https://github.com/Zygo/bees
Copyright & License
Copyright 2015-2025 Zygo Blaxell bees@furryterror.org.
GPL (version 3 or later).