btrfs-check(8) - Man pages

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.

NOTE:
Even though the filesystem checker requires a device argument, it
scans for all devices belonging to the same filesystem, thus it
should not cause a difference using different devices of the same
filesystem. Furthermore btrfs-check(8) will automatically choose the
good mirror, thus as long as there is a good copy for metadata, it
will not report such case as an error.

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 affect
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

--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 workaround
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.

-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.

DANGEROUS OPTIONS
--repair
enable the repair mode and attempt to fix problems where possible

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.

--force
allow work on a mounted filesystem and skip mount checks. 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.

NOTE:
It is possible to run with --repair but on a mounted filesys-
tem that will most likely lead to a corruption unless the
filesystem is in a quiescent state which may not be possible
to guarantee.

This option also skips the delay and warning in the repair mode
(see --repair).

DEPRECATED OR REMOVED OPTIONS
--clear-space-cache v1|v2
completely remove the free space cache of the given version