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docs: toxic extents and btrfs send

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Update documentation of toxic extent / slow backref workaround.

Add notes about btrfs send kernel bugs and incremental send failures.

Signed-off-by: Zygo Blaxell <bees@furryterror.org>
This commit is contained in:
Zygo Blaxell
2018-11-06 00:56:06 -05:00
parent 688d0dc014
commit 19859b0a0d
3 changed files with 27 additions and 28 deletions
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docs/btrfs-kernel.md
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@@ -38,19 +38,23 @@ Unfixed kernel bugs (as of 4.14.71):
`rsync` is copying from, while `rsync` will rename the new file over
the old file to replace it.
* **btrfs send** has various problems when bees is deduping RO snapshots,
especially if the snapshot is used as a parent for incremental send.
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.
* **Slow backrefs** (aka toxic extents): Under certain conditions,
if the number of references to a single shared extent grows too high,
the kernel consumes more and more CPU while holding 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. In the bees log, such blocks are labelled 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.
Older kernels:
* Older kernels have various data corruption and deadlock/hang issues
that are no longer listed here, and older kernels are missing important
features such as `LOGICAL_INO_V2`. Using an older kernel is not
recommended.