btrfs scrub <subcommand> <args>
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.
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 btrfs scrub start for more.
- 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] <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.
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.
do not background and print scrub statistics when finished
print separate statistics for each device of the filesystem (-B only) at the end
run in read-only mode, do not attempt to correct anything, can be run on a read-only filesystem
raw print mode, print full data instead of summary
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
- -c <ioprio_class>
set IO priority class (see
ionice(1)manpage) if the IO scheduler configured for the device supports ionice. This is not supported byg BFQ or Kyber but is not supported by mq-deadline.
- -n <ioprio_classdata>
set IO priority classdata (see
(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.
print separate statistics for each device of the filesystem
print all raw statistics without postprocessing as returned by the status ioctl
print all numbers raw values in bytes without the B suffix
print human friendly numbers, base 1024, this is the default
select the 1024 base for the following options, according to the IEC standard
select the 1000 base for the following options, according to the SI standard
show sizes in KiB, or kB with --si
show sizes in MiB, or MB with --si
show sizes in GiB, or GB with --si
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
btrfs scrub returns a zero exit status if it succeeds. Non zero is returned in case of failure:
scrub couldn’t be performed
there is nothing to resume
scrub found uncorrectable errors
btrfs is part of btrfs-progs. Please refer to the documentation at https://btrfs.readthedocs.io.