[dm-devel] Use LVM on /dev/mapper/diskname

[malahal at us.ibm.com]

Tore Anderson [tore at linpro.no] wrote:
> * S. J. van Harmelen
> 
> > I have a couple of LUN's that are handled by the multipath driver.
> > This works great, but now I want to use LVM so I can take snapshots
> > of one LUN to another LUN.
> > 
> > Can someone tell me how to do this?
> > 
> > When I do pvcreate /dev/mapper/diskname I get an error that the disk
> > is already part of an volume group. But I sertainly did not configure
> > that.
> > 
> > Can it be that the multipath driver of devmapper did that?
> 
> If you have a PV signature on the volume, LVM might have used one of the
> paths instead of using the multipath'ed device under /dev/mapper.  You
> should be able to check this by running the command "pvs" - it should
> list all the detected PVs on your system.

If LVM uses single pathed disks like /dev/sda, then multipath can't use
the same path to make a multipathed device, right? My guess is that
/dev/mapper/multipath-name is indeed part of the LVM in your case. I
would like to know what happened otherwise. Thank you.

--Malahal.