[linux-lvm] device mapper Q

[linux-lvm] device mapper Q

[Dieter Stueken]

maybe this is more dm related than LVM2, but I could not
finger out where to find more info/help....

after I turned to LVM2, I noticed, that the VG-specific
subdirectory below /dev/<VG>/<LV> now contains symlinks
to /dev/mapper/<VG>-<LV> devices.

Those devices also appear as /dev/dm-<N> again.

Q: who creates those links? For any reason I'm missing
one of these link. Should I create it by hand?

If mounting an LV, which path should I prefer?

I used to use EXT2 labels and entered the LVs to
the /etc/fstab using LABEL=<NAME>. With LVM2/DM the
kernel now locates them as /dev/dm_<N> instead of
/dev/<VG>/<LV>. Does it matter?

Is there any more DM background information around besides
http://sources.redhat.com/dm/, or do I have to analyze
the C-code to learn more about it?

Dieter.
-- 
Dieter St�ken, con terra GmbH, M�nster
     stueken@conterra.de
     http://www.conterra.de/
     (0)251-7474-501

Re: [linux-lvm] device mapper Q

[Alasdair G Kergon]

On Fri, Nov 19, 2004 at 08:33:19PM +0100, Dieter Stueken wrote:
> Q: For any reason I'm missing
> one of these link. Should I create it by hand?

vgmknodes fixes things

Boot-up scripts should include vgscan --mknodes.

LVM2 will be updated at some point to recognise /dev/mapper/vg-lv
names on the command line, but it doesn't yet - you must always
use vg/lv.

dm-<minor_number> in sysfs/block is something I have no time for
but seems pretty harmless.

But /dev/dm* sounds quite unnecessary.

devfs puts things into /dev/mapper automatically: udev should
be configured to do the same - but it seems it can't because
that would mean userspace call outs running scripts at critical 
times when they are not permitted to block (e.g. must not allocate 
any memory) or else could lock up the whole machine.
So everything should use libdevmapper (or equivalent), often
via dmsetup, to keep /dev/mapper up-to-date.

Alasdair
-- 
agk@redhat.com