[linux-lvm] Identifying useable block devices

Karel Zak kzak at redhat.com
Fri Jan 17 09:29:21 UTC 2014

On Fri, Jan 17, 2014 at 08:54:15AM +0100, Peter Rajnoha wrote:
> On 01/16/2014 10:03 PM, Paul B. Henson wrote:
> >> Peter Rajnoha
> >> Sent: Wednesday, January 15, 2014 9:43 PM
> >>
> >> This applies only if the symlinks/nodes are created directly with
> >> libdevmapper - which is the old classical way.
> >> If udev is used to manage /dev
> >> content for dm/lvm devices then the /dev/vg/vol is a symlink to /dev/dm-X
> >> (also /dev/mapper/vg-vol is a symlink to /dev/dm-X). And dm-X is not a
> > 
> > Hmm, yah, I see on my RHEL6 boxes that is indeed the case, as opposed to my
> > older RHEL5 boxes.
> > 
> >> As for mount, findmnt and other utilities from util-linux - they all use
> >> one common code which can handle this situation (e.g. if you call
> >> "findmnt /dev/vg/vol", it shows you /dev/mapper/vg-vol on output, not
> >> the dm-X the /dev/vg/vol is referencing actually).
> > 
> > IIRC, this thread started when somebody was being advised *not* to use the
> > /dev/mapper path, but rather to use the /dev/vg path. If there is special
> > case handling in util-linux, why isn't it returning what is being advertised
> > as the preferable path to use rather than the one that is said to be
> > internal and not to be referenced?
> Well, that is probably an RFE for util-linux to give priority to
> /dev/<vgname>/<lvname> instead of /dev/mapper/<vgname>-<lvname>
> if it's available...
> Karel, would that be possbible for you to add? (though I expect
> you probably don't want to add any additional and specific logic
> for dm devices :) )

I have spent years to move from /dev/dm-N to /dev/mapper/<name> ... :-)

Note that /dev/mapper/<name> paths are in many cases composed from
/sys/block/dm-<N>/dm/name and this sysfs file has been added (by
Milan) just to support conversion from random DM paths (or devno) to
the canonicalized /dev/mapper/<name>.

How I can convert maj:min or /dev/dm-N to /dev/vg/vol path? Is the
volume group name (always) in /sys/block/dm-<N>/dm/name and (always)
separated by "-" ?

It also seems that this is LVM specific, for non-LVM devices (for 
example LUKS) we want to use /dev/mapper/<name>.


 Karel Zak  <kzak at redhat.com>

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