# btrfs-check

## NAME



           btrfs-check - check or repair a btrfs filesystem
    


## SYNOPSIS



           **btrfs check**[options] _< device>_
    


## DESCRIPTION



           The filesystem checker is used to verify structural integrity of a
           filesystem 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 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 correctly connected together.
    
           There are several cross checks that can detect wrong reference
           counts of shared extents, backreferences, missing extents of
           inodes, directory and inode connectivity etc.
    
           The amount of memory required can be high, depending on the size
           of the filesystem, similarly the run time. Check the modes that
           can also affect that.
    


## SAFE OR ADVISORY OPTIONS



           -b|--backup
               use the first valid set of backup roots stored in the
               superblock
    
               This can be combined with _--super_ if some of the superblocks
               are damaged.
    
           --check-data-csum
               verify checksums of data blocks
    
               This expects that the filesystem is otherwise OK, and is
               basically an offline _scrub_ that does not repair data from
               spare copies.
    
           --chunk-root _< bytenr>_
               use the given offset _bytenr_ for the chunk tree root
    
           -E|--subvol-extents _< subvolid>_
               show extent state for the given subvolume
    
           -p|--progress
               indicate progress at various checking phases
    
           -Q|--qgroup-report
               verify qgroup accounting and compare against filesystem
               accounting
    
           -r|--tree-root _< bytenr>_
               use the given offset _bytenr_ for the tree root
    
           --readonly
               (default) run in read-only mode, this option exists to calm
               potential panic when users are going to run the checker
    
           -s|--super _< superblock>_
               use 'superblock’th superblock copy, valid values are 0, 1 or 2
               if the respective superblock offset is within the device size
    
               This can be used to use a different starting point if some of
               the primary superblock is damaged.
    
           --clear-space-cache v1|v2
               completely wipe all free space cache of given type
    
               For free space cache _v1_ , the _clear_cache_ kernel mount option
               only rebuilds the free space cache for block groups that are
               modified while the filesystem is 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
               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.
    
           --clear-ino-cache
               remove leftover items pertaining to the deprecated inode map
               feature
    


## DANGEROUS OPTIONS



           --repair
               enable the repair mode and attempt to fix problems where
               possible
    
                   **Note**
                   there’s a warning and 10 second delay when this option is
                   run without _--force_ to give users a chance to think twice
                   before running repair, the warnings in documentation have
                   shown to be insufficient
    
           --init-csum-tree
               create a new checksum tree and recalculate checksums in all
               files
    
                   **Note**
                   Do not blindly use this option to fix checksum mismatch
                   problems.
    
           --init-extent-tree
               build the extent tree from scratch
    
                   **Note**
                   Do not use unless you know what you’re doing.
    
           --mode _< MODE>_
               select mode of operation regarding memory and IO
    
               The _MODE_ can be one of:
    
               _original_
                   The metadata are read into memory and verified, thus the
                   requirements are high on large filesystems and can even
                   lead to out-of-memory conditions. The possible workaround
                   is to export the block device over network to a machine
                   with enough memory.
    
               _lowmem_
                   This mode is supposed to address the high memory
                   consumption at the cost of increased IO when it needs to
                   re-read blocks. This may increase run time.
    
                       **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.
    
               This option also skips the delay and warning in the repair
               mode (see _--repair_).
    


## EXIT STATUS



           **btrfs check** returns a zero exit status if it succeeds. Non zero is
           returned in case of failure.
    


## AVAILABILITY



           **btrfs** is part of btrfs-progs. Please refer to the btrfs wiki
           **http://btrfs.wiki.kernel.org******for further details.


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