[dm-devel] [PATCH 08/10] kpartx: use partition UUID for non-DM devices

[Martin Wilck]

On Fri, 2017-05-05 at 23:30 +0100, Alasdair G Kergon wrote:
> On Sat, May 06, 2017 at 12:05:57AM +0200, Martin Wilck wrote:
> > Introduce a "fake" UUID for these devices to make sure kpartx
> > deletes only devices it had created previously. Otherwise kpartx
> > might e.g. delete LVM LVs that are inside a device it is trying
> > to delete partitions for. It seems to be wiser to make sure the
> > user delete these manually before running "kpartx -d".
> > 
> > With the fake UUID in place, we can re-introduce the UUID check
> > for non-DM device that was removed in the earlier patch
> > "kpartx: dm_remove_partmaps: support non-dm devices".
> > 
> > This disables also deletion of partition mappings created
> > by earlier versions of kpartx. If kpartx has been updated after
> > partition mappings were created, the "-f" flag can be used
> > to force delting these partitions, too.
> 
>  
> Indeed - always supply a uuid for every dm device created.
> Add a standard prefix to the beginning of it, ideally "KPARTX-" to be
> consistent with other dm tools ("LVM-", "CRYPT-" etc.) if you're
> able to break compatibility, but "part" is still unique and
> adequate.  
> (Stacked devices can end up with a stack of prefixes - only the
> leftmost one
> counts.)

As noted in my reply to Hannes' mail, I'm using the prefix "part%u-",
the same prefix that kpartx currently used for partitions of DM
devices.

The full format e.g. for /dev/mapper/loop0p1 is 

"part1-7:0-kpartx-Wh5pYvM7uc60hh74" 

and correspondingly, the "fake UUID" used for /dev/loop0 would be 

      "7:0-kpartx-Wh5pYvM7uc60hh74"

If you don't like the fact that this starts with "7:0", changing this
to e.g. "KPARTX-7.0-$something" is no problem as long as patch isn't
applied.

The last part ("Wh...") is a hard-coded string, just to avoid
accidental conflicts. I was wondering whether it would make sense to
some sort of encoded "host ID" to make this UUID truly "universal". My
personal opinion is that this would be over-engineered, because these
UUID will be visible only only on the host that called "kpartx -a" in
the first place.

Cheers,
Martin

-- 
Dr. Martin Wilck <mwilck at suse.com>, Tel. +49 (0)911 74053 2107
SUSE Linux GmbH, GF: Felix Imendörffer, Jane Smithard, Graham Norton
HRB 21284 (AG Nürnberg)