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btrfs-kernel.md

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Recommended kernel version

Linux 4.14.34 or later.

A Brief List Of Btrfs Kernel Bugs

Recent kernel bug fixes:

  • 4.14.29: WARN_ON(ref->count < 0) in fs/btrfs/backref.c triggers almost once per second. The WARN_ON is incorrect, and is now removed.

Unfixed kernel bugs (as of 4.14.71):

  • Bad filesystem destroying interactions with other Linux block layers: bcache and lvmcache can fail spectacularly, and apparently only do so while running bees. This is definitely a kernel bug, either in btrfs or the lower block layers. Avoid using bees with these tools unless your filesystem is disposable and you intend to debug the kernel.

  • Compressed data corruption is possible when using the fallocate system call to punch holes into compressed extents that contain long runs of zeros. The bug results in intermittent corruption during reads, but due to the bug, the kernel might sometimes mistakenly determine data is duplicate, and deduplication will corrupt the data permanently. This bug also affects compressed kvm raw images with the discard feature on btrfs or any compressed file where fallocate -d or fallocate -p has been used.

  • Deadlock when simultaneously using the same files in dedupe and rename. There is no way for bees to reliably know when another process is about to rename a file while bees is deduping it. In the rsync case, bees will dedupe the new file rsync is creating using the old file rsync is copying from, while rsync will rename the new file over the old file to replace it.

Minor kernel problems with workarounds:

  • Slow backrefs (aka toxic extents): If the number of references to a single shared extent within a single file grows above a few thousand, the kernel consumes CPU for minutes at a time while holding various locks that block access to the filesystem. bees avoids this bug by measuring the time the kernel spends performing LOGICAL_INO operations and permanently blacklisting any extent or hash involved where the kernel starts to get slow. Inside bees, such blocks are known as 'toxic' hash/block addresses.

  • FILE_EXTENT_SAME is arbitrarily limited to 16MB. This is less than 128MB which is the maximum extent size that can be created by defrag, prealloc, or filesystems without the compress-force mount option. bees avoids feedback loops this can generate while attempting to replace extents over 16MB in length.