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

[S. J. van Harmelen]

On Mon, 2007-11-19 at 09:22 -0800, malahal at us.ibm.com wrote:
> 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.

I already send this in a reply to the list, but will mail it to you
directly in case you missed it:

I executed the pvs command and it did find a few partitions. I use iSCSI
to share the LUN's to a XenSource server (I use the /dev/mapper/diskname
as sharing point for iSCSI), and XenSource creates the PV's
and LV's in order to provision VM's. Didn't think of that, but that's on
the disk of course.

Here's the output of pvs:

root at storage:~# pvs
  /dev/sdc: read failed after 0 of 4096 at 0: Input/output error
  /dev/sdd: read failed after 0 of 4096 at 0: Input/output error
  /dev/sdf: read failed after 0 of 4096 at 0: Input/output error
  /dev/sdi: read failed after 0 of 4096 at 0: Input/output error
  PV         VG                                                 Fmt
Attr PSize   PFree  
  /dev/sde   VG_XenStorage-755d6649-e770-6f01-5e17-fd7f8dcd8651 lvm2 a-
199.99G 185.49G

The four disks that give a "read failed" error are the redundant disk,
so this is no problem.

Now is there any way to configure snapshots for these already created
LVM partitions? Or do I have to whipe the disk clean (rather not do
that, but if I have to it can be done), then create the pv's and lv's
and then share the lv's true iSCSI?

In that case XenSource will again create pv's and lv's in the shared lv.
Ain't that a problem?

Sander

> 
> --Malahal.
> 
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