Ubuntu Manpage: btrfstune - tune various filesystem parameters

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