Re: udev and md/dm

[Joel Becker <Joel.Becker@oracle.com>]

On Sat, Jun 16, 2007 at 04:32:49AM +0200, Kay Sievers wrote:
> On 6/16/07, Joel Becker <Joel.Becker@oracle.com> wrote:
> >        Why does udev ignore md* and dm-* devices for persistent naming?
> >I was utterly surprised to find that an ext3 on /dev/md0 or /dev/dm-0
> >won't show up in /dev/disk/by-uuid.  Given that dm and md devices can
> >change names, isn't this something we'd really like to have?
> 
> Sure, they just have their own rules files. Both have specific
> requirements/lifetime-rules and plug into "change" events.

	I'm looking at rhel5, debian, and raw udev-105, and I can't find
anywhere in the rules that use vol_id to map md or dm devices to
/dev/disk/by-label and /dev/disk/by-uuid.  The vol_id using files
(persistent.rules) skips those devices.  I can see why perhaps you'd say
"loop and ramdisk may be garbage", but dm*, md*, and similar are
expected persistent environment things.  And their names can certainly
change.
	I ask because I am looking at implementing a robust "how to
determine what device is meant by UUID xxxxx".  Robust in the face of
catching the raid1 and multipath and so on.  So /dev/disk/by-uuid/xxxxx
wouldn't point to /dev/sdd1 if it really should be /dev/md0.  I had
hoped udev + vol_id would do this for me, as vol_id seems to have
explicit ordering to make this work right.  But the udev rules skip
checking uuids and labels on the aggregate devices.
	Please, show me where I'm wrong :-)


Joel

-- 

"The question of whether computers can think is just like the question
 of whether submarines can swim."
	- Edsger W. Dijkstra

Joel Becker
Principal Software Developer
Oracle
E-mail: joel.becker@oracle.com
Phone: (650) 506-8127

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