Provided by: btrfs-progs_6.6.3-1.2_amd64 
NAME
btrfs-scrub - scrub btrfs filesystem, verify block checksums
SYNOPSIS
btrfs scrub <subcommand> <args>
DESCRIPTION
Scrub is a pass over all filesystem data and metadata and verifying the checksums. If a
valid copy is available (replicated block group profiles) then the damaged one is
repaired. All copies of the replicated profiles are validated.
NOTE:
Scrub is not a filesystem checker (fsck) and does not verify nor repair structural
damage in the filesystem. It really only checks checksums of data and tree blocks, it
doesn't ensure the content of tree blocks is valid and consistent. There's some
validation performed when metadata blocks are read from disk (Tree checker) but it's
not extensive and cannot substitute full btrfs-check(8) run.
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 "IOBandwidthReadMax=/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 [-BdrRf] <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
-R raw print mode, print full data instead of summary
-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
-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.
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)