BTRFS-CHECK(8) BTRFS BTRFS-CHECK(8)
NAME
btrfs-check - check or repair a btrfs filesystem
SYNOPSIS
btrfs check [options] <device>
DESCRIPTION
The filesystem checker is used to verify structural integrity of a
filesystem and attempt to repair it if requested. It is recommended to
unmount the filesystem prior to running the check, but it is possible
to start checking a mounted filesystem (see --force).
By default, btrfs check will not modify the device but you can reaffirm
that by the option --readonly.
btrfsck is an alias of btrfs check command and is now deprecated.
WARNING:
Do not use --repair unless you are advised to do so by a developer
or an experienced user, and then only after having accepted that no
fsck successfully repair all types of filesystem corruption. E.g.
some other software or hardware bugs can fatally damage a volume.
The structural integrity check verifies if internal filesystem objects
or data structures satisfy the constraints, point to the right objects
or are correctly connected together.
There are several cross checks that can detect wrong reference counts
of shared extents, backreferences, missing extents of inodes, directory
and inode connectivity etc.
The amount of memory required can be high, depending on the size of the
filesystem, similarly the run time. Check the modes that can also af-
fect that.
SAFE OR ADVISORY OPTIONS
-b|--backup
use the first valid set of backup roots stored in the superblock
This can be combined with --super if some of the superblocks are
damaged.
--check-data-csum
verify checksums of data blocks
This expects that the filesystem is otherwise OK, and is basi-
cally an offline scrub that does not repair data from spare
copies.
--chunk-root <bytenr>
use the given offset bytenr for the chunk tree root
-E|--subvol-extents <subvolid>
show extent state for the given subvolume
-p|--progress
indicate progress at various checking phases
-Q|--qgroup-report
verify qgroup accounting and compare against filesystem account-
ing
-r|--tree-root <bytenr>
use the given offset 'bytenr' for the tree root
--readonly
(default) run in read-only mode, this option exists to calm po-
tential panic when users are going to run the checker
-s|--super <N>
use Nth superblock copy, valid values are 0, 1 or 2 if the re-
spective superblock offset is within the device size
This can be used to use a different starting point if some of
the primary superblock is damaged.
--clear-space-cache v1|v2
completely wipe all free space cache of given type
For free space cache v1, the clear_cache kernel mount option
only rebuilds the free space cache for block groups that are
modified while the filesystem is mounted with that option. Thus,
using this option with v1 makes it possible to actually clear
the entire free space cache.
For free space cache v2, the clear_cache kernel mount option de-
stroys the entire free space cache. This option, with v2 pro-
vides an alternative method of clearing the free space cache
that doesn't require mounting the filesystem.
--clear-ino-cache
remove leftover items pertaining to the deprecated inode map
feature
DANGEROUS OPTIONS
--repair
enable the repair mode and attempt to fix problems where possi-
ble
NOTE:
There's a warning and 10 second delay when this option is run
without --force to give users a chance to think twice before
running repair, the warnings in documentation have shown to
be insufficient
--init-csum-tree
create a new checksum tree and recalculate checksums in all
files
WARNING:
Do not blindly use this option to fix checksum mismatch prob-
lems.
--init-extent-tree
build the extent tree from scratch
WARNING:
Do not use unless you know what you're doing.
--mode <MODE>
select mode of operation regarding memory and IO
The MODE can be one of:
original
The metadata are read into memory and verified, thus the
requirements are high on large filesystems and can even
lead to out-of-memory conditions. The possible work-
around is to export the block device over network to a
machine with enough memory.
lowmem This mode is supposed to address the high memory consump-
tion at the cost of increased IO when it needs to re-read
blocks. This may increase run time.
NOTE:
lowmem mode does not work with --repair yet, and is still
considered experimental.
--force
allow work on a mounted filesystem. Note that this should work
fine on a quiescent or read-only mounted filesystem but may
crash if the device is changed externally, e.g. by the kernel
module. Repair without mount checks is not supported right now.
This option also skips the delay and warning in the repair mode
(see --repair).
EXIT STATUS
btrfs check returns a zero exit status if it succeeds. Non zero is re-
turned in case of failure.
AVAILABILITY
btrfs is part of btrfs-progs. Please refer to the documentation at
https://btrfs.readthedocs.io or wiki http://btrfs.wiki.kernel.org for
further information.
SEE ALSO
mkfs.btrfs(8), btrfs-scrub(8), btrfs-rescue(8)
6.2 Feb 28, 2023 BTRFS-CHECK(8)
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