Re: A Plumber???s Wish List for Linux

[Dave Chinner <david@fromorbit.com>]

On Mon, Oct 10, 2011 at 09:09:37AM -0400, Theodore Tso wrote:
> 
> On Oct 10, 2011, at 7:18 AM, David Sterba wrote:
> 
> > "Resetting the UUID on btrfs isn't a quick-and-easy thing - you
> > have to walk the entire tree and change every object. We've got
> > a bad-hack in meego that uses btrfs-debug-tree and changes the
> > UUID while it runs the entire tree, but it's ugly as hell."
> 
> Changing the UUID is going to be harder for ext4 as well, once we
> integrate metadata checksums. 

And for XFS, we're modifying the on-disk format to encode the UUID
into every single piece of metadata in the filesystem. Hence
changing it entails a similar problem to btrfs - an entire
filesystem metadata RMW cycle.

> So while it makes sense to have
> on-line ways of updating labels for mounted file systems it
> probably makes muchness sense to support it for UUIDs.
                     ^^^^ less
Agreed.

> I suspect what it means in practice is that it will be useful for
> file systems to provide fs image copying tools that also generate
> a new UUID while you're at it, for use by IT administrators and
> embedded systems manufacturers.

Yup. xfs_admin already provides an interface for offline
modification of the UUID for XFS filesytems. I.e. clone the
filesytem using xfs_copy, then run xfs_admin -U generate <clone> to
generate a new uuid in the cloned copy before you mount the
clone....

Cheers,

Dave.
-- 
Dave Chinner
david@fromorbit.com