Ubuntu Manpage: btrfs-scrub - scrub btrfs filesystem, verify block checksums

NAME

       btrfs-scrub - scrub btrfs filesystem, verify block checksums

SYNOPSIS

       btrfs scrub <subcommand> <args>

DESCRIPTION

btrfs scrub is used to scrub a mounted btrfs filesystem, which will read all data and
metadata blocks from all devices and verify checksums. Automatically repair corrupted
blocks if there’s a correct copy available.

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 but it’s not extensive
and cannot substitute full btrfs check run.

The user is supposed to run it manually or via a periodic system service. The recommended
period is a month but could be less. The estimated device bandwidth utilization is about
80% on an idle filesystem. The IO priority class is by default idle so background scrub
should not significantly interfere with normal filesystem operation. The IO scheduler set
for the device(s) might not support the priority classes though.

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 scrub start for more.

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.

       resume [-BdqrR] [-c <ioprio_class> -n <ioprio_classdata>] <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 [-BdqrRf] [-c <ioprio_class> -n <ioprio_classdata>] <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 is performed by default for block group profiles with redundancy.

           The default IO priority of scrub is the idle class. The priority can be configured
           similar to the ionice(1) syntax using -c and -n options. Note that not all IO
           schedulers honor the ionice settings.

           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

           -c <ioprio_class>
               set IO priority class (see ionice(1) manpage)

           -n <ioprio_classdata>
               set IO priority classdata (see ionice(1) manpage)

           -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

           -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

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