btrfstune

BTRFSTUNE(8) Btrfs Manual BTRFSTUNE(8)

NAME
btrfstune - tune various filesystem parameters

SYNOPSIS
btrfstune [options] [...]

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 .

OPTIONS
-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.

-r
Enable extended inode refs (hardlink limit per file in a directory is 65536), enabled
by mkfs feature extref. Since kernel 3.7.

-x
Enable skinny metadata extent refs (more efficient representation of extents), enabled
by mkfs feature skinny-metadata. Since kernel 3.10.

-n
Enable no-holes feature (more efficient representation of file holes), enabled by mkfs
feature no-holes. Since kernel 3.14.

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

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

-U
Change fsid to UUID.

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.

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

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.

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

COMPATIBILITY NOTE
This tool exists for historical reasons but is still in use today. The functionality is
about to be merged to the main tool someday and btrfstune will become deprecated and
removed afterwards.