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docs: various gotcha updates

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Fixing the obviously wrong and out of date stuff.

Signed-off-by: Zygo Blaxell <bees@furryterror.org>
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Zygo Blaxell 2023-02-25 03:11:30 -05:00
parent c354e77634
commit c92ba117d8
1 changed files with 7 additions and 8 deletions
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docs/gotchas.md
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@ -85,10 +85,6 @@ The shutdown procedure performs these steps:
Balances Balances
-------- --------
First, read [`LOGICAL_INO` and btrfs balance WARNING](btrfs-kernel.md).
bees will suspend operations during a btrfs balance to work around
kernel bugs.
A btrfs balance relocates data on disk by making a new copy of the A btrfs balance relocates data on disk by making a new copy of the
data, replacing all references to the old data with references to the data, replacing all references to the old data with references to the
new copy, and deleting the old copy. To bees, this is the same as any new copy, and deleting the old copy. To bees, this is the same as any
@ -138,7 +134,9 @@ the beginning.
Each time bees dedupes an extent that is referenced by a snapshot, Each time bees dedupes an extent that is referenced by a snapshot,
the entire metadata page in the snapshot subvol (16KB by default) must the entire metadata page in the snapshot subvol (16KB by default) must
be CoWed in btrfs. This can result in a substantial increase in btrfs be CoWed in btrfs. Since all references must be removed at the same
time, this CoW operation is repeated in every snapshot containing the
duplicate data. This can result in a substantial increase in btrfs
metadata size if there are many snapshots on a filesystem. metadata size if there are many snapshots on a filesystem.
Normally, metadata is small (less than 1% of the filesystem) and dedupe Normally, metadata is small (less than 1% of the filesystem) and dedupe
@ -215,17 +213,18 @@ Other Gotchas
filesystem while `LOGICAL_INO` is running. Generally the CPU spends filesystem while `LOGICAL_INO` is running. Generally the CPU spends
most of the runtime of the `LOGICAL_INO` ioctl running the kernel, 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 second so on a single-core CPU the entire system can freeze up for a second
during operations on toxic extents. during operations on toxic extents. Note this only occurs on older
kernels. See [the slow backrefs kernel bug section](btrfs-kernel.md).
* If a process holds a directory FD open, the subvol containing the * If a process holds a directory FD open, the subvol containing the
directory cannot be deleted (`btrfs sub del` will start the deletion directory cannot be deleted (`btrfs sub del` will start the deletion
process, but it will not proceed past the first open directory FD). process, but it will not proceed past the first open directory FD).
`btrfs-cleaner` will simply skip over the directory *and all of its `btrfs-cleaner` will simply skip over the directory *and all of its
children* until the FD is closed. bees avoids this gotcha by closing children* until the FD is closed. bees avoids this gotcha by closing
all of the FDs in its directory FD cache every 10 btrfs transactions. all of the FDs in its directory FD cache every btrfs transaction.
* If a file is deleted while bees is caching an open FD to the file, * If a file is deleted while bees is caching an open FD to the file,
bees continues to scan the file. For very large files (e.g. VM bees continues to scan the file. For very large files (e.g. VM
images), the deletion of the file can be delayed indefinitely. images), the deletion of the file can be delayed indefinitely.
To limit this delay, bees closes all FDs in its file FD cache every To limit this delay, bees closes all FDs in its file FD cache every
10 btrfs transactions. btrfs transaction.