Re: Udev integration for device-mapper and its subsystems.

[Milan Broz <mbroz@redhat.com>]

Kay Sievers wrote:
> On Wed, Apr 29, 2009 at 04:45, Alasdair G Kergon <agk@redhat.com> wrote:
>>> Also DM allows pretty stupid free-text device names, and names
>> We took the view there was no need for any restrictions beyond
>> '/' in the kernel and it was a matter for userspace to choose
>> the namespace.
> 
> And these days are over. /dev is not the thing anymore where everybody
> can mess around.

We had problems with some multipath vendors which named storage device
paths using strange names/chars (according to LUN name, from hw).

Also dmraid sets uses names collected from proprietary metadata.

Change this means change to several tools. And there are probably still
reasons to name devices according to hw provided names.

So if there is name limitation, we need to create some mapping which
is acceptable both for udev and administrators/users...


> Sure rename them. But if you want the device node renamed, rename the
> kernel devices too.

Sure, but it will have consequences which must be solved too.

E.g. the dm uuid can be max. 128 characters, I am sure that we can use that
in kernel for internal device name.
(uuid is not just plain UUID string, it includes prefix of subsystem,
like LVM- MPATH- etc.)

But how many userspace tools expects such long name in /sys/block?



People are using udev but also lvm, dmaid, multipath, kpartx, cryptsetup, etc.

We need to find some way how to fix that and not break these systems.

Maybe the symlink to /dev/dm-X is first step. This will switch dm to using udev,
what is the primary goal now, thought.

and (later?),

- introduce mandatory uuid for dm device (or disable rename for devices which
have no uuid set and keep the old dm-X name for them?).

- use dm uuid, so the internal kernel device name will be persistent
(and avoid dm-X where X is dynamically allocated number)
(I think udev rules should be still the same here if written properly.)

This requires kernel changes outside DM, like remove device name limit
in partitioning code.

- fix tools to "understand" the new names
  * long name in /sys limitation
  * fix mapping of not udev-valid characters
  * probably some tweaks for tools which sometimes prefer
    to display symlinks instead of kernel name (if symlink exists)

(I mean e.g. lvm user expect device like /dev/VG/LV1 (symlink) for PV report
and not /dev/dm-LVM-0124-9438-1238-8129.
If there is way to avoid these tweaks and keep /dev/<internal kernel name>
then of course use that:-)
(I mean tools like blkid & Co.)

Please also note that clustered LVM can complicate things, the device
name (created node in dev) must be the same on all nodes in cluster etc.

Milan
--
mbroz@redhat.com

p.s.
I feel that this discussion heads to flame who is dreaming and slept last
years... please, no:-)