btrfstune man page
btrfstune — tune various filesystem parameters
btrfstune [options] <dev> [<dev>...]
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 enabled on a mounted filesystem. Please refer to the respective section in btrfs(5).
- -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.
(since kernel: 3.7)
Enable extended inode refs (hardlink limit per file in a directory is 65536), enabled by mkfs feature extref. -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). -n:: (since kernel: 3.14)
Enable no-holes feature (more efficient representation of file holes), enabled by mkfs feature no-holes. -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.
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.
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.
btrfstune returns 0 if no error happened, 1 otherwise.
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.
btrfs(5), btrfs-balance(8), mkfs.btrfs(8)