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

[Alasdair G Kergon <agk@redhat.com>]

On Tue, Apr 28, 2009 at 11:25:31PM +0200, Kay Sievers wrote:
> Don't know, the name is probably not the right thing, because it can
> be changed, The uuid might fit better.

But why would changing the name be a problem?
What is fundamental here is the device number - that's all.
E.g. Anything in userspace that opens /dev/something has to check that
what it actually opened still has the same device number, if it cares
about avoiding races.

If we can extend this field in-kernel and put the user's preferred device name
in there we'd gain the immediate big advantage of error messages produced
throughout the kernel using a name that the sysadmin can translate into
something they understand much more easily.  "dm-vg1-lvol1" instead of "dm-13".
And this would be a big help to people I believe.  (Although the dm name
can be very long, on most systems it's still quite short.  The uuid on the
other hand is long and typically randomly-generated and meaningless to the
sysadmin.)

Then any userspace packages would by default pick up a meaningful name - 
something considerably more persistent and useful than dm-X.

So my latest proposal would be:
1) Change the kernel to allow longer names for block devices
2) Change dm to insert dm-<name> into this field instead of dm-X
3) Add an in-kernel interface to let dm change this name on request by the user
4) Adjust udev rules to create nodes as /dev/dm-<name> 
   (actually I'd still argue that /dev/subsystem/name is a better approach)
5) Change all dm-based userspace tools to handle /dev/dm-* 
6) Change anything if there is anything in userspace that cares about those
/dev names getting changed on a live object and needs to react differently to a
CHANGE uevent.

And eventually perhaps:
7) Phase out /dev/mapper (in a suitably-compatible way as usual so we don't
   break anything)
8) Consider whether people are ready to stomach the phasing out of /dev/vg/lv
   or whether that should move to /dev/lvm/vg/lv so it doesn't pollute the
   flat /dev namespace.  (LVM2 has always worked with just vg/lv and simply
   treats '/dev/' as an optional prefix, not a path.)

9) Also consider whether there's a better prefix than dm- to use on these
devices.  It's too easily confused with md.  map- perhaps.  Other ideas?

Alasdair
-- 
agk@redhat.com