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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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22
docs/gotchas.md
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@@ -78,22 +78,16 @@ Other Gotchas
-------------
* bees avoids the [slow backrefs kernel bug](btrfs-kernel.md) by
measuring the time required to perform `LOGICAL_INO` operations. If an
extent requires over 10 seconds to perform a `LOGICAL_INO` then bees
blacklists the extent and avoids referencing it in future operations.
In most cases, fewer than 0.1% of extents in a filesystem must be
avoided this way. This results in short write latency spikes of up
to and a little over 10 seconds as btrfs will not allow writes to the
measuring the time required to perform `LOGICAL_INO` operations.
If an extent requires over 0.1 kernel CPU seconds to perform a
`LOGICAL_INO` ioctl, then bees blacklists the extent and avoids
referencing it in future operations. In most cases, fewer than 0.1%
of extents in a filesystem must be avoided this way. This results
in short write latency spikes as btrfs will not allow writes to the
filesystem while `LOGICAL_INO` is running. Generally the CPU spends
most of the runtime of the `LOGICAL_INO` ioctl running the kernel,
so on a single-core CPU the entire system can freeze up for a few
seconds at a time.
* Load managers that send a `SIGSTOP` to the bees process to throttle
CPU usage may affect the `LOGICAL_INO` timing mechanism, causing extents
to be incorrectly labelled 'toxic'. This will cause a small reduction
of dedupe hit rate. Slow and heavily loaded disks can trigger the same
effect if `LOGICAL_INO` takes too long due to IO latency.
so on a single-core CPU the entire system can freeze up for a second
during operations on toxic extents.
* If a process holds a directory FD open, the subvol containing the
directory cannot be deleted (`btrfs sub del` will start the deletion