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. See also
section SEEDING DEVICE in btrfs(5).

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