btrfs-progs/Documentation/btrfstune.asciidoc

btrfstune(8)
============

NAME
----
btrfstune - tune various filesystem parameters

SYNOPSIS
--------
*btrfstune* [options] <dev> [<dev>...]

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 <UUID>::
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.

SEE ALSO
--------
`mkfs.btrfs`(8)