Bug#542422: [linux-lvm] Re: Bug#542422: dmsetup: I would like to have speaking device names back

Peter Rajnoha prajnoha at redhat.com
Tue Sep 8 08:41:08 UTC 2009

On 08/19/2009 05:15 PM, Bastian Blank wrote:
> On Wed, Aug 19, 2009 at 04:50:09PM +0200, Marc Haber wrote:
>> Since the last update, my crypto devices are recognized by the system as
>> /dev/dm-xx, for example in fsck and in df's output:
>> /dev/dm-15             12G   11G  565M  96% /mnt/home
>> /dev/dm-13            7,9G  7,0G  532M  94% /mnt/usr
>> /dev/dm-14            3,0G  1,4G  1,5G  50% /mnt/var
>> lrwxrwxrwx  1 root root      8 19. Aug 09:06 home -> ../dm-15
>> lrwxrwxrwx  1 root root      8 19. Aug 09:06 usr -> ../dm-13
>> lrwxrwxrwx  1 root root      8 19. Aug 09:06 var -> ../dm-14
>> I'd like them to show up as /dev/mapper/home, /dev/mapper/usr and
>> /dev/mapper/var again.
> This means that the real devices needs to be named this way as mount
> always dereferences symlinks.
> Patch which does this against the Debian package is attached.
> Alasdair: Does Red Hat solve that problem somehow?
> Bastian

Just to explain why we decided to do it this way in LVM upstream:

The (very) early version of the rules were discussed with udev team
so it would be correct from udev point of view as well. The discussion
led us to a solution where we had to use /dev/dm-* as nodes and
/dev/mapper/* as symlinks, citing Kay Severs:

"In general we do not want any unneeded disconnect from kernel names
and /dev names, and dm block devices should stay as /dev/dm-* device

"Please do not rename kernel devices, they should match the kernel names.
Only create SYMLINK+= to the kernel names..."

(you can find the whole discussion at http://markmail.org/message/bj4zkjo2peeocnhq)

We're just trying to comply with those udev requirements...

Yes, there are some problems associated with the utilities using
/dev/mapper nodes, but, as Alasdair says, this should be corrected
there directly. We have no other way, either we break 'udev laws'
or we break a few utilities. But we would like to do this correctly,
so I'm voting for the update of those utilities (the grub2 problem
mentioned somewhere here in this bz -- I'm already preparing
a proposal for the grub team to deal with this issue).

It's quite painful now, I know, but once done correctly and having
all depending things fixed, we will have a proper solution that
everybody will be happy with.


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