Re: Ext4 external journal UUID mismatch?

[Theodore Ts'o <tytso@mit.edu>]

On Mon, Jul 16, 2012 at 07:30:10PM +0930, Kevin Shanahan wrote:
> > The lookup of the journal by UUID (instead of relying on the "device
> > hint" in the superblock) _should_ be handled by mount, but I don't
> > recall if we ever got a mount.ext4 to handle this or not. It would
> > also be possible for the "fast e2fsck" check to verify the journal
> > UUID before mounting the filesystem, but again I'm not sure if this
> > is done yet, and I can't check right now.
> 
> Ok, thanks. For now I can ask LVM to make the major/minor number
> persistent.
> 
> Assuming it's not already done, do you think adding the UUID lookup
> would be a reasonable project for a newbie or is it likely to be a bit
> complicated?

We have a mount option.  You can specify it via a mount option, i.e.,

   -o journal_dev=0xFC02

It would be nice if the mount program (in util-linux) did a uuid
lookup using the blkid library if there was a mount option string of
the form "journal_dev=lookup".

I would certainly support getting a feature like that into mount
command, but it would be up to the maintainer of the util-linux
package to approve the specific patch.  Since it would require linking
in the ext2fs library, or putting in enough knowledge of the ext3/4
superblock to fetch the external journal uuid, I could imagine the
util-linux maintainer insisting that this be done in a helper program
(i.e., /sbin/mount.ext3 and /sbin/mount.ext4).  So it could
potentially get a little complicated, just to warn you ahead of time.

Regards,

					- Ted