Re: udev and md/dm

[Andrey Borzenkov <arvidjaar@mail.ru>]

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On Saturday 16 June 2007, Joel Becker wrote:
> 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.

at least device mapper already has entries under /dev/mapper; I has been using 
it for multipathing only so far, and multipath can be configured to create 
persistent names there. I presume other device mapper modules can be told to 
create persistent names too.

For mdadm you can specify persistent names based on UUID or array name as 
well.

I agree that having everything in one place simplifies things.

> 	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. 

It depends what in imply by "implementing". It is definitely possible to write 
such script; moreover, it is quite possible that blkid already does what you 
need:

{pts/0}% sudo blkid -l -t UUID="72973377-c299-4ca6-ae94-2022b7dca58f"
/dev/sda2: UUID="72973377-c299-4ca6-ae94-2022b7dca58f" LABEL="/" 
TYPE="reiserfs"

-andrey

> 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



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