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Scrub is a pass over all filesystem data and metadata and verifying the
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checksums. If a valid copy is available (replicated block group profiles) then
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the damaged one is repaired. All copies of the replicated profiles are validated.
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.. note::
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Scrub is not a filesystem checker (fsck) and does not verify nor repair
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structural damage in the filesystem. It really only checks checksums of data
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and tree blocks, it doesn't ensure the content of tree blocks is valid and
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consistent. There's some validation performed when metadata blocks are read
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2023-12-08 17:01:42 +00:00
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from disk (:doc:`Tree-checker`) but it's not extensive and cannot substitute
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full :doc:`btrfs-check` run.
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The user is supposed to run it manually or via a periodic system service. The
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recommended period is a month but it could be less. The estimated device bandwidth
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utilization is about 80% on an idle filesystem.
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2023-06-28 17:55:08 +00:00
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The scrubbing status is recorded in :file:`/var/lib/btrfs/` in textual files named
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*scrub.status.UUID* for a filesystem identified by the given UUID. (Progress
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state is communicated through a named pipe in file *scrub.progress.UUID* in the
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same directory.) The status file is updated every 5 seconds. A resumed scrub
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will continue from the last saved position.
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Scrub can be started only on a mounted filesystem, though it's possible to
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scrub only a selected device. See :ref:`btrfs scrub start<man-scrub-start>` for more.
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.. _scrub-io-limiting:
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Bandwidth and IO limiting
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^^^^^^^^^^^^^^^^^^^^^^^^^
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.. note::
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The ``ionice(1)`` may not be generally supported by all IO schedulers and
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the options to :command:`btrfs scrub start` may not work as expected.
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In the past when the `CFQ IO scheduler
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<https://en.wikipedia.org/wiki/Completely_fair_queueing>`__ was generally used
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the ``ionice(1)`` syscalls set the priority to *idle* so the IO would not
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interfere with regular IO. Since the kernel 5.0 the CFQ is not available.
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The IO scheduler known to support that is `BFQ
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<https://docs.kernel.org/block/bfq-iosched.html>`__, but first read the
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documentation before using it!
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For other commonly used schedulers like `mq-deadline
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<https://docs.kernel.org/block/blk-mq.html>`__ it's recommended to use
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*cgroup2 IO controller* which could be managed by e.g. *systemd*
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(documented in ``systemd.resource-control``). However, starting scrub like that
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is not yet completely straightforward. The IO controller must know the physical
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device of the filesystem and create a slice so all processes started from that
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belong to the same accounting group.
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.. code-block:: bash
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$ systemd-run -p "IOBandwidthReadMax=/dev/sdx 10M" btrfs scrub start -B /
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Since linux 5.14 it's possible to set the per-device bandwidth limits in a
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BTRFS-specific way using files :file:`/sys/fs/btrfs/FSID/devinfo/DEVID/scrub_speed_max`.
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This setting is not persistent, lasts until the filesystem is unmounted.
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Currently set limits can be displayed by command :ref:`btrfs scrub
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limit<man-scrub-limit>`.
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.. code-block:: bash
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$ echo 100m > /sys/fs/btrfs/9b5fd16e-1b64-4f9b-904a-74e74c0bbadc/devinfo/1/scrub_speed_max
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$ btrfs scrub limit /
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UUID: 9b5fd16e-1b64-4f9b-904a-74e74c0bbadc
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Id Limit Path
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-- --------- --------
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1 100.00MiB /dev/sdx
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