BTRFS-SCRUB(8) BTRFS BTRFS-SCRUB(8)
NAME
btrfs-scrub - scrub btrfs filesystem, verify block checksums
SYNOPSIS
btrfs scrub <subcommand> <args>
DESCRIPTION
Scrub is a validation pass over all filesystem data and metadata that
detects data checksum errors, basic super block errors, basic metadata
block header errors, and disk read errors.
Scrub is done on a per-device base, if a device is specified to btrfs
scrub start, then only that device will be scrubbed. Although btrfs
will also try to read other device to find a good copy, if the mirror
on that specified device failed to be read or pass verification.
If a path of btrfs is specified to btrfs scrub start, btrfs will scrub
all devices in parallel.
On filesystems that use replicated block group profiles (e.g. RAID1),
read-write scrub will also automatically repair any damage by copying
verified good data from one of the other replicas.
Such automatic repair is also carried out when reading metadata or data
from a read-write mounted filesystem.
WARNING:
As currently implemented, setting the NOCOW file attribute (by
chattr +C) on a file implicitly enables NODATASUM. This means that
while metadata for these files continues to be validated and
corrected by scrub, the actual file data is not.
Furthermore, btrfs does not currently mark missing or failed disks
as unreliable, so will continue to load-balance reads to potentially
damaged replicas. This is not a problem normally because damage is
detected by checksum validation, but because NOCOW files are not
protected by checksums, btrfs has no idea which mirror is good thus
it can return the bad contents to the user space tool.
Detecting and recovering from such failure requires manual
intervention.
Notably, systemd sets +C on journals by default, and libvirt ≥ 6.6
sets +C on storage pool directories by default. Other applications
or distributions may also set +C to try to improve performance.
NOTE:
Scrub is not a filesystem checker (fsck, btrfs-check(8)). It can
only detect filesystem damage using the checksum validation, and it
can only repair filesystem damage by copying from other known good
replicas.
btrfs-check(8) performs more exhaustive checking and can sometimes
be used, with expert guidance, to rebuild certain corrupted
filesystem structures in the absence of any good replica.
NOTE:
Read-only scrub on a read-write filesystem will cause some writes
into the filesystem.
This is due to the design limitation to prevent race between marking
block group read-only and writing back block group items.
To avoid any writes from scrub, one has to run read-only scrub on
read-only filesystem.
The user is supposed to run it manually or via a periodic system
service. The recommended period is a month but it could be less. The
estimated device bandwidth utilization is about 80% on an idle
filesystem.
The scrubbing status is recorded in /var/lib/btrfs/ in textual files
named scrub.status.UUID for a filesystem identified by the given UUID.
(Progress state is communicated through a named pipe in file
scrub.progress.UUID in the same directory.) The status file is updated
every 5 seconds. A resumed scrub will continue from the last saved
position.
Scrub can be started only on a mounted filesystem, though it's possible
to scrub only a selected device. See btrfs scrub start for more.
Bandwidth and IO limiting
NOTE:
The ionice(1) may not be generally supported by all IO schedulers
and the options to btrfs scrub start may not work as expected.
In the past when the CFQ IO scheduler was generally used the ionice(1)
syscalls set the priority to idle so the IO would not interfere with
regular IO. Since the kernel 5.0 the CFQ is not available.
The IO scheduler known to support that is BFQ, but first read the
documentation before using it!
For other commonly used schedulers like mq-deadline it's recommended to
use cgroup2 IO controller which could be managed by e.g. systemd
(documented in systemd.resource-control). However, starting scrub like
that is not yet completely straightforward. The IO controller must know
the physical device of the filesystem and create a slice so all
processes started from that belong to the same accounting group.
$ systemd-run -p "IOReadBandwidthMax=/dev/sdx 10M" btrfs scrub start -B /
Since linux 5.14 it's possible to set the per-device bandwidth limits
in a BTRFS-specific way using files
/sys/fs/btrfs/FSID/devinfo/DEVID/scrub_speed_max. This setting is not
persistent, lasts until the filesystem is unmounted. Currently set
limits can be displayed by command btrfs scrub limit.
$ echo 100m > /sys/fs/btrfs/9b5fd16e-1b64-4f9b-904a-74e74c0bbadc/devinfo/1/scrub_speed_max
$ btrfs scrub limit /
UUID: 9b5fd16e-1b64-4f9b-904a-74e74c0bbadc
Id Limit Path
-- --------- --------
1 100.00MiB /dev/sdx
SUBCOMMAND
cancel <path>|<device>
If a scrub is running on the filesystem identified by path or
device, cancel it.
If a device is specified, the corresponding filesystem is found
and btrfs scrub cancel behaves as if it was called on that
filesystem. The progress is saved in the status file so btrfs
scrub resume can continue from the last position.
limit [options] <path>
Show or set scrub limits on devices of the given filesystem.
Options
-d|--devid DEVID
select the device by DEVID to apply the limit
-l|--limit SIZE
set the limit of the device to SIZE (size units with
suffix), or 0 to reset to unlimited
-a|--all
apply the limit to all devices
--raw print all numbers raw values in bytes without the B
suffix
--human-readable
print human friendly numbers, base 1024, this is the
default
--iec select the 1024 base for the following options, according
to the IEC standard
--si select the 1000 base for the following options, according
to the SI standard
--kbytes
show sizes in KiB, or kB with --si
--mbytes
show sizes in MiB, or MB with --si
--gbytes
show sizes in GiB, or GB with --si
--tbytes
show sizes in TiB, or TB with --si
resume [-BdqrR] <path>|<device>
Resume a cancelled or interrupted scrub on the filesystem
identified by path or on a given device. The starting point is
read from the status file if it exists.
This does not start a new scrub if the last scrub finished
successfully.
Options
see scrub start.
start [options] <path>|<device>
Start a scrub on all devices of the mounted filesystem
identified by path or on a single device. If a scrub is already
running, the new one will not start. A device of an unmounted
filesystem cannot be scrubbed this way.
Without options, scrub is started as a background process. The
automatic repairs of damaged copies are performed by default for
block group profiles with redundancy. No-repair can be enabled
by option -r.
Options
-B do not background and print scrub statistics when
finished
-d print separate statistics for each device of the
filesystem (-B only) at the end
-r run in read-only mode, do not attempt to correct
anything, can be run on a read-only filesystem
Note that a read-only scrub on a read-write filesystem
can still cause writes into the filesystem due to some
internal limitations. Only a read-only scrub on a
read-only filesystem can avoid writes from scrub.
-R raw print mode, print full data instead of summary
--limit <limit>
set the scrub throughput limit for each device.
If the scrub is for the whole filesystem, it's the same
as btrfs scrub limit --all --limit <value>. If the scrub
is for a single device, it's the same as btrfs scrub
limit --devid <devid> -l <value>.
The value is bytes per second, and accepts the usual KMGT
suffixes. After the scrub is finished, the throughput
limit will be reset to the old value of each device.
-f force starting new scrub even if a scrub is already
running, this can useful when scrub status file is
damaged and reports a running scrub although it is not,
but should not normally be necessary
Deprecated options
The priority settings work only with certain schedulers, in
particular they don't work with the most common one mq-deadline.
-c <ioprio_class>
set IO priority class (see ionice(1) manual page) if the
IO scheduler configured for the device supports ionice.
This is only supported by BFQ or Kyber but is not
supported by mq-deadline. Please read the section about
IO limiting.
-n <ioprio_classdata>
set IO priority classdata (see ionice(1) manpage)
-q (deprecated) alias for global -q option
status [options] <path>|<device>
Show status of a running scrub for the filesystem identified by
path or for the specified device.
If no scrub is running, show statistics of the last finished or
cancelled scrub for that filesystem or device. The status is
read from the file located in /var/lib/btrfs.
Options
-d print separate statistics for each device of the
filesystem
-R print all raw statistics without postprocessing as
returned by the status ioctl
--raw print all numbers raw values in bytes without the B
suffix
--human-readable
print human friendly numbers, base 1024, this is the
default
--iec select the 1024 base for the following options, according
to the IEC standard
--si select the 1000 base for the following options, according
to the SI standard
--kbytes
show sizes in KiB, or kB with --si
--mbytes
show sizes in MiB, or MB with --si
--gbytes
show sizes in GiB, or GB with --si
--tbytes
show sizes in TiB, or TB with --si
A status on a filesystem without any error looks like the
following:
# btrfs scrub start /
# btrfs scrub status /
UUID: 76fac721-2294-4f89-a1af-620cde7a1980
Scrub started: Wed Apr 10 12:34:56 2023
Status: running
Duration: 0:00:05
Time left: 0:00:05
ETA: Wed Apr 10 12:35:01 2023
Total to scrub: 28.32GiB
Bytes scrubbed: 13.76GiB (48.59%)
Rate: 2.75GiB/s
Error summary: no errors found
With some errors found:
Error summary: csum=72
Corrected: 2
Uncorrectable: 72
Unverified: 0
• Corrected -- number of bad blocks that were repaired from
another copy
• Uncorrectable -- errors detected at read time but not possible
to repair from other copy
• Unverified -- transient errors, first read failed but a retry
succeeded, may be affected by lower layers that group or split
IO requests
• Error summary -- followed by a more detailed list of errors
found
• csum -- checksum mismatch
• super -- super block errors, unless the error is fixed
immediately, the next commit will overwrite superblock
• verify -- metadata block header errors
• read -- blocks can't be read due to IO errors
It's possible to set a per-device limit via file
sysfs/fs/btrfs/FSID/devinfo/scrub_speed_max. In that case the
limit is printed on the Rate: line if option -d is specified, or
without it on a single-device filesystem. Read more about tat
in section about scrub IO limiting.
Rate: 989.0MiB/s (limit 1.0G/s)
On a multi-device filesystem with at least one device limit the
overall stats cannot print the limit without -d so there's a not
that some limits are set:
Rate: 36.37MiB/s (some device limits set)
EXIT STATUS
btrfs scrub returns a zero exit status if it succeeds. Non zero is
returned in case of failure:
1 scrub couldn't be performed
2 there is nothing to resume
3 scrub found uncorrectable errors
AVAILABILITY
btrfs is part of btrfs-progs. Please refer to the documentation at
https://btrfs.readthedocs.io.
SEE ALSO
mkfs.btrfs(8)
6.16.1 September 10, 2025 BTRFS-SCRUB(8)
btrfs scrub \- scrub btrfs filesystem, verify block checksums