NAME
----
-btrfs-check - check or repair an unmounted btrfs filesystem
+btrfs-check - check or repair a btrfs filesystem
SYNOPSIS
--------
-----------
The filesystem checker is used to verify structural integrity of a filesystem
-and attempt to repair it if requested. The filesystem must be unmounted.
+and attempt to repair it if requested. It is recommended to unmount the
+filesystem prior to running the check, but it is possible to start checking a
+mounted filesystem (see '--force').
By default, *btrfs check* will not modify the device but you can reaffirm that
by the option '--readonly'.
*btrfsck* is an alias of *btrfs check* command and is now deprecated.
-WARNING: Do not use '--repair' unless you are advised to by a developer, an
-experienced user or accept the fact that 'fsck' cannot possibly fix all sorts
-of damage that could happen to a filesystem because of software and hardware
-bugs.
+WARNING: Do not use '--repair' unless you are advised to do so by a developer
+or an experienced user, and then only after having accepted that no 'fsck'
+successfully repair all types of filesystem corruption. Eg. some other software
+or hardware bugs can fatally damage a volume.
The structural integrity check verifies if internal filesystem objects or
data structures satisfy the constraints, point to the right objects or are
--check-data-csum::
verify checksums of data blocks
+
-This expects that the filesystem is otherwise
-OK, so this is basically and offline 'scrub' but does not repair data from
-spare copies.
+This expects that the filesystem is otherwise OK, and is basically and offline
+'scrub' but does not repair data from spare copies.
--chunk-root <bytenr>::
use the given offset 'bytenr' for the chunk tree root
-p|--progress::
indicate progress at various checking phases
---qgroup-report::
+-Q|--qgroup-report::
verify qgroup accounting and compare against filesystem accounting
-r|--tree-root <bytenr>::
mounted with that option. Thus, using this option with 'v1' makes it possible
to actually clear the entire free space cache.
+
-For free space cache 'v2', the 'clear_cache' kernel mount option does destroy
-the entire free space cache. This option with 'v2' provides an alternative
+For free space cache 'v2', the 'clear_cache' kernel mount option destroys
+the entire free space cache. This option, with 'v2' provides an alternative
method of clearing the free space cache that doesn't require mounting the
filesystem.
NOTE: 'lowmem' mode does not work with '--repair' yet, and is still considered
experimental.
+--force::
+allow work on a mounted filesystem. Note that this should work fine on a
+quiescent or read-only mounted filesystem but may crash if the device is
+changed externally, eg. by the kernel module. Repair without mount checks is
+not supported right now.
+
EXIT STATUS
-----------
*btrfs check* returns a zero exit status if it succeeds. Non zero is