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.
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>
This does not start a new scrub if the last scrub finished successfully.
see scrub start.
start [-BdqrRf] [-c <ioprio_class> -n <ioprio_classdata>] <path>|<device>
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.
status [options] <path>|<device>
If no scrub is running, show statistics of the last finished or cancelled scrub for that filesystem or device.