linux-kernel - Re: A Plumber???s Wish List for Linux

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Message-ID: <20111013002839.GL3159@dastard>
Date:	Thu, 13 Oct 2011 11:28:39 +1100
From:	Dave Chinner <david@...morbit.com>
To:	Theodore Tso <tytso@....EDU>
Cc:	dave@...os.cz, Hugo Mills <hugo-lkml@...fax.org.uk>,
	Kay Sievers <kay.sievers@...y.org>,
	Alan Cox <alan@...rguk.ukuu.org.uk>,
	linux-kernel@...r.kernel.org, lennart@...ttering.net,
	harald@...hat.com, david@...ar.dk, greg@...ah.com,
	Chris Mason <chris.mason@...cle.com>,
	Btrfs mailing list <linux-btrfs@...r.kernel.org>
Subject: Re: A Plumber???s Wish List for Linux

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@...morbit.com
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