Re: Changes in device-mapper and LVM2 that can affect grub's functionality

[Peter Rajnoha]

On 09/09/2009 12:26 PM, Felix Zielcke wrote:
> What's the point in having the /dev/dm-X devices at all?
> Does anything use them?

...nothing uses them and nothing ever should! These names are not
stable (the number depends on the activation sequence of those
devices). Not usable for anything, really, just to keep udev guys
happy :)

Actually, we wanted to abandon these nodes first, so they would never
be visible under /dev. And keep the old way instead.

But when we discussed this with Kay Sievers from udev:

"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
nodes."

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

"Sure, but there is still not enough reason to be different from the
kernel name. You support rename, that should never happen, that a device
node needs to be renamed, if the kernel does not change the device name."

"Sounds fine, as long as the /dev names match the kernel devices."

"There is the rule, that kernel block device names and /dev names match,
and the kernel log shows device names which translate directly to the
primary device nodes. DM is not an exception here, all block device
behave like that , and we like to continue that, and not make needless
rules just to be special here."

...and so on and on...

(the whole discussion is at http://markmail.org/message/bj4zkjo2peeocnhq)

> Currently all symlinks are ignored.
> If we use the target of the /dev/mapper/* symlinks, i.e. a /dev/dm-X
> device this would at least with the default Debian initrd not work and I
> doubt the responding persons for this will change this ever. Even
> root=UUID= isn't working for LVM devices, because only the root LV is
> activated and not all inside the initrd.
> 
> If we would use the symlink itself for root= it could break if there
> were symlinks which aren't inside the initrd too.
> 
> I personally don't like this change at all.
> Why not just remove the dm-X devices and make the /dev/mapper/ ones the
> only and real ones?

...we would like to, but...

> Maybe the udev maintainers just prefer cryptic numbers for every real
> device and only accept symlinks for descriptive ones.

Yes, that's the case.

OK, I'll try to talk with Kay again and discuss all the problems that
this "udev law" brings in real-life situations for dm devices and
everything using it.

I just needed your opinion, too, thanks! So maybe we have one more
argument against such layout for udev guys...

Peter