BTRFSTUNE(8)                          Btrfs Manual                         BTRFSTUNE(8)

NAME
       btrfstune - tune various filesystem parameters

SYNOPSIS
       btrfstune [options] <device> [<device>...]

DESCRIPTION
       btrfstune can be used to enable, disable, or set various filesystem parameters.
       The filesystem must be unmounted.

       The common usecase is to enable features that were not enabled at mkfs time.
       Please make sure that you have kernel support for the features. You can find a
       complete list of features and kernel version of their introduction at
       https://btrfs.wiki.kernel.org/index.php/Changelog#By_feature . Also, the manual
       page mkfs.btrfs(8) contains more details about the features.

       Some of the features could be also enabled on a mounted filesystem by other
       means. Please refer to the FILESYSTEM FEATURES in btrfs(5).

OPTIONS
       -f
           Allow dangerous changes, e.g. clear the seeding flag or change fsid. Make
           sure that you are aware of the dangers.

       -m
           (since kernel: 5.0)

           change fsid stored as metadata_uuid to a randomly generated UUID, see also
           -U

       -M <UUID>
           (since kernel: 5.0)

           change fsid stored as metadata_uuid to a given UUID, see also -U

           The metadata_uuid is stored only in the superblock and is a backward
           incompatible change. The fsid in metadata blocks remains unchanged and is
           not overwritten, thus the whole operation is significantly faster than -U.

           The new metadata_uuid can be used for mount by UUID and is also used to
           identify devices of a multi-device filesystem.

       -n
           (since kernel: 3.14)

           Enable no-holes feature (more efficient representation of file holes),
           enabled by mkfs feature no-holes.

       -r
           (since kernel: 3.7)

           Enable extended inode refs (hardlink limit per file in a directory is
           65536), enabled by mkfs feature extref.

       -S <0|1>
           Enable seeding on a given device. Value 1 will enable seeding, 0 will
           disable it.

           A seeding filesystem is forced to be mounted read-only. A new device can be
           added to the filesystem and will capture all writes keeping the seeding
           device intact.

               Warning
               Clearing the seeding flag on a device may be dangerous. If a
               previously-seeding device is changed, all filesystems that used that
               device will become unmountable. Setting the seeding flag back will not
               fix that.

               A valid usecase is seeding device as a base image. Clear the seeding
               flag, update the filesystem and make it seeding again, provided that
               it’s OK to throw away all filesystems built on top of the previous base.

       -u
           Change fsid to a randomly generated UUID or continue previous fsid change
           operation in case it was interrupted.

       -U <UUID>
           Change fsid to UUID in all metadata blocks.

           The UUID should be a 36 bytes string in printf(3) format
           "%08x-%04x-%04x-%04x-%012x". If there is a previous unfinished fsid change,
           it will continue only if the UUID matches the unfinished one or if you use
           the option -u.

           All metadata blocks are rewritten, this may take some time, but the final
           filesystem compatibility is unaffected, unlike -M.

               Warning
               Cancelling or interrupting a UUID change operation will make the
               filesystem temporarily unmountable. To fix it, rerun btrfstune -u and
               let it complete.

       -x
           (since kernel: 3.10)

           Enable skinny metadata extent refs (more efficient representation of
           extents), enabled by mkfs feature skinny-metadata.

           All newly created extents will use the new representation. To completely
           switch the entire filesystem, run a full balance of the metadata. Please
           refer to btrfs-balance(8).

EXIT STATUS
       btrfstune returns 0 if no error happened, 1 otherwise.

COMPATIBILITY NOTE
       This deprecated tool exists for historical reasons but is still in use today.
       Its functionality will be merged to the main tool, at which time btrfstune will
       be declared obsolete and scheduled for removal.

SEE ALSO
       btrfs(5), btrfs-balance(8), mkfs.btrfs(8)

Btrfs v4.20.1                          01/23/2019                          BTRFSTUNE(8)

usage: btrfstune [options] device
Tune settings of filesystem features on an unmounted device

Options:
  change feature status:
        -r          enable extended inode refs (mkfs: extref, for hardlink limits)
        -x          enable skinny metadata extent refs (mkfs: skinny-metadata)
        -n          enable no-holes feature (mkfs: no-holes, more efficient sparse file representation)
        -S <0|1>    set/unset seeding status of a device
  uuid changes:
        -u          rewrite fsid, use a random one
        -U UUID     rewrite fsid to UUID
        -m          change fsid in metadata_uuid to a random UUID
                    (incompat change, more lightweight than -u|-U)
        -M UUID     change fsid in metadata_uuid to UUID
  general:
        -f          allow dangerous operations, make sure that you are aware of the dangers
        --help      print this help