# man > btrfs-scrub(8) [_BTRFS-SCRUB_(8)](https://www.chedong.com/phpMan.php/man/BTRFS-SCRUB/8/markdown) BTRFS [_BTRFS-SCRUB_(8)](https://www.chedong.com/phpMan.php/man/BTRFS-SCRUB/8/markdown) ## NAME btrfs-scrub - scrub btrfs filesystem, verify block checksums ## SYNOPSIS **btrfs scrub ** ## 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 en‐ sure 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)](https://www.chedong.com/phpMan.php/man/btrfs-check/8/markdown)_ run. The user is supposed to run it manually or via a periodic system service. The recommended pe‐ riod 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 se‐ lected device. See _btrfs_ _scrub_ _start_ for more. ### Bandwidth and IO limiting ### NOTE: The [**ionice(1)](https://www.chedong.com/phpMan.php/man/ionice/1/markdown) **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)](https://www.chedong.com/phpMan.php/man/ionice/1/markdown) **syscalls set the pri‐ ority 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_ _con‐_ _troller_ which could be managed by e.g. _systemd_ (documented in **systemd.resource-control**). How‐ ever, 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 "IOBandwidthReadMax=/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 persis‐ tent, lasts until the filesystem is unmounted. Currently set limits can be displayed by com‐ mand _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 | 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] 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] | 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] | Start a scrub on all devices of the mounted filesystem identified by _path_ or on a sin‐ gle _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 _**_ set IO priority class (see [**ionice(1)](https://www.chedong.com/phpMan.php/man/ionice/1/markdown) **manual page) if the IO scheduler config‐ ured 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 _**_ set IO priority classdata (see [**ionice(1)](https://www.chedong.com/phpMan.php/man/ionice/1/markdown) **manpage) ### -q (deprecated) ### status [options] | Show status of a running scrub for the filesystem identified by _path_ or for the speci‐ fied _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 af‐ fected 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 com‐ mit 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/dev‐** **info/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 fail‐ ure: **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 _. ## SEE ALSO [_mkfs.btrfs(8)](https://www.chedong.com/phpMan.php/man/mkfs.btrfs/8/markdown)_ 6.6.3 Mar 31, 2024 [_BTRFS-SCRUB_(8)](https://www.chedong.com/phpMan.php/man/BTRFS-SCRUB/8/markdown)