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 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 "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 [-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)