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

[Milan Broz <mbroz@redhat.com>]

Kay Sievers wrote:
> On Tue, Apr 28, 2009 at 20:45, Alasdair G Kergon <agk@redhat.com> wrote:
>> 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 no size limit anymore.

I think that limit comes from 16 chars for block device name
in register_blkdev() in kernel.
And also DISK_NAME_LEN is 32 (for genhd->disk_name, used by bdevname())...

(The dm device name has limit 128. How to set internal kernel name
to properly to match dm name?)

(And DM device name is already in  /sys/block/<dm-X>/dm/name,
udev rules can use that value for symlink, no timing problem here.)


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

This will break many userspace applications... If the uuid is set,
userspace must use that uuid in all dm-ioctl calls to address device.
And not all users of device mapper library set UUID, because it is
not mandatory (an example is cryptsetup).
(So it is not quick switch because of need to fix all applications to work
properly and set uuid for all dm devices... IMHO)

Milan
--
mbroz@redhat.com