man > btrfs-scrub(8)

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 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 en‐
          sure 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 pe‐
       riod 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 se‐
       lected 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  pri‐
       ority  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 con‐
       troller which could be managed by e.g. systemd (documented in systemd.resource-control). How‐
       ever, 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  persis‐
       tent, lasts until the filesystem is unmounted.  Currently set limits can be displayed by com‐
       mand 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 sin‐
              gle 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 config‐
                     ured 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 speci‐
              fied 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  af‐
                fected 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 com‐
                  mit 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/dev‐
              info/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 fail‐
       ure:

       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)


6.6.3                                       Mar 31, 2024                              BTRFS-SCRUB(8)