btrfs-check(8)
NAME
btrfs-check - check or repair an unmounted 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. The filesystem must
be unmounted.
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 adviced to by a developer, an
experienced user or accept the fact that fsck cannot possibly fix
all sorts of damage that could happen to a filesystem because of
software and hardware bugs.
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, backrefrences, missing extents of inodes, directory
and inode connectivity etc.
The amount of memory required can be high, depending on the size of the
filesystem, smililarly the run time.
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, so this is
basically and offline scrub but does not repair data from spare
coipes.
--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
--qgroup-report
verify qgroup accounting and compare against filesystem accounting
-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
potential panic when users are going to run the checker
-s|--super <superblock>
use 'superblock'th superblock copy, valid values are 0, 1 or 2 if
the respective 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
--init-csum-tree
create a new checksum tree and recalculate checksums in all files
Note
Do not blindly use this option to fix checksum mismatch
problems.
--init-extent-tree
build the extent tree from scratch
Note
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 and lowmem. The original mode is
mostly unoptimized regarding memory consumpption and can lead to
out-of-memory conditions on large filesystems. The possible
workaround is to export the block device over network to a machine
with enough memory. The low memory mode is supposed to address the
memory consumption, at the cost of increased IO when it needs to
re-read blocks when needed. This may increase run time.
Note
lowmem mode does not work with --repair yet, and is still
considered experimental.
EXIT STATUS
btrfs check returns a zero exit status if it succeeds. Non zero is returned in case of failure.
AVAILABILITY
btrfs is part of btrfs-progs. Please refer to the btrfs wiki http://btrfs.wiki.kernel.org for further details.
SEE ALSO
mkfs.btrfs(8), btrfs-scrub(8), btrfs-rescue(8)
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