Re: Use LVM on /dev/mapper/diskname

[Tore Anderson <tore@linpro.no>]

* 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 this was the problem, you can avoid it by instructing LVM to not scan
SCSI devices directly, by adding a line in /etc/lvm/lvm.conf like this:

device {
  filter = [ "a|^/dev/mapper/.*|", "r|.*|" ]
}

This will make LVM only look at devices in /dev/mapper/ as possible PV
candidates, ignoring all other devices.  Beware if you have a PV on the
internal drives though, you might want to have something like this
instead in that case (if that PV is on /dev/sda2, for instance):

  filter = [ "a|^/dev/mapper/.*|", "a|^/dev/sda2$|", "r|.*|" ]

Make sure lvm.conf makes its way into the initramfs if you start the LVM
stuff there.  I know Ubuntu doesn't include this file by default, at least.

Regards
-- 
Tore Anderson