btrfs-progs/Documentation/btrfs-scrub.rst

btrfs-scrub(8)
==============

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
------------

**btrfs** is part of btrfs-progs.
Please refer to the btrfs wiki http://btrfs.wiki.kernel.org for
further details.

SEE ALSO
--------

``mkfs.btrfs(8)``,
``ionice(1)``