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

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

On Tue, Apr 28, 2009 at 07:41:10PM +0200, Kay Sievers wrote:
> Please do not rename kernel devices, they should match the kernel
> names. Only create SYMLINK+= to the kernel names, or change the kernel
> to ceate the proper names, which will not work I guess, so please
> leave them as they are today.
 
> 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.
 
The /dev/dm-* devices have been a support headache for us.  I don't know
how they first sneaked into some distros but I maintain that dm-* are
internal kernel names only of use for debugging purposes and the average
sysadmin should never have to encounter them.  If I had my way they
would never have become visible to userspace.  (Unfortunately the size
restriction on the field prevents us from replacing them with our
preferred names within the kernel, unless that's changed since
I last looked.)

There is nothing fundamental about dm-*, they are meaningless to the
user, absolutely not persistent across boots and tools may need to
change what they mean whenever there is some reconfiguration.  Their
presence causes no end of problems as they 'leak' out into initrds,
fstabs, error messages etc.  On a normal, functioning system, absolutely
nothing in userspace should ever reference them.

The namespace for dm devices is currently *only* under /dev/mapper.

If people *really* think dm devices need to appear directly in a flat
namespace in /dev (why?) then we should drop the /dev/mapper directory
and move them to /dev/mapper-vg1-lvol1 for example.  (The whole strength
of udev is that it makes it easy to do something like that, in fact.)

Alternatively, we could see about making dm uuids mandatory and
go for something like /dev/dm-<uuid>.

Alasdair
-- 
agk@redhat.com