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 :command:`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 :command:`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
:command:`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, :doc:`btrfs-check`). It can only detect filesystem damage using the checksum validation, and it can only repair filesystem damage by copying from other known good replicas.
:doc:`btrfs-check` 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 :file:`/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 :ref:`btrfs scrub start<man-scrub-start>` for more.
.. duplabel:: scrub-io-limiting
Note
The :manref:`ionice(1)` may not be generally supported by all IO schedulers and the options to :command:`btrfs scrub start` may not work as expected.
In the past when the CFQ IO scheduler was generally used the :manref:`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 :file:`/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 :ref:`btrfs scrub limit<man-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