On Wed, 21 May 2014 21:14:07 -0500
Eric Sandeen <sandeen@redhat.com> wrote:

> >> (Random aside: why does btrfs support online fs relabeling, anyway?)
> >>
> >> -Eric
> > 
> >   Online you mean when mounted ?
> 
> Yep - I'm just not sure who would ever want to do that.
> 
> Aren't labels primarly used for mounting, during the mount process?

Well if you want to change the label of your root filesystem, how else would
you go about that? If Btrfs did not support this, then only while booted from a
rescue LiveCD? That's quite a bit more involved and not even always feasible
in case of remote machines with no IPMI or similar management.

Extfs supports online change of the volume label as well, albeit probably not
via a sysfs file, but with the tune2fs utility:

  tune2fs -L <name> /dev/blockdevice

-- 
With respect,
Roman