[linux-lvm] LVM snapshots in a iSCSI and XenSource environment

S. J. van Harmelen svh at dds.nl
Tue Nov 20 17:56:18 UTC 2007

On Tue, 2007-11-20 at 09:29 -0800, malahal at us.ibm.com wrote:
> S. J. van Harmelen [svh at dds.nl] wrote:
> > 
> > Now my thoughts where to take snapshots on the storage server. So that
> > would mean I would take a snapshot of lv_storage. This should not be a
> > problem. But what if virtual machine 2 crashes and has to be restored?
> > 
> > In that case I cannot restore lv_storage with a dd command, because then
> > all the data will be restored (lv_virtualmachine1 and
> > lv_virtualmachine2, because they are both on lv_storage). So is it
> > possible to "restore" only lv_virtualmachine2 when I took a snapshot of
> > lv_storage?
> I see, it is possible but a bit cumbersome. You should be able to run
> LVM on the storage server on the restored/old data (LVM on top of LVM??) and
> get the data for the lv_virtualmachine2 from the storage server to
> XenSource (ftp/tar whatever method). Then, restore that data (tar
> extract) at XenSource.
> If things are NOT changing at the XenSource LVM, you could just "dd" on
> the crashed LV, but things may go really bad otherwise.
> Any better method(s)???

Well of course this could also be done from the Xen server. Then there
is no need for LVM on the storage server and the lv_storage part can be
left out of this picture.

I could then create a second iSCSI target on a second LUN to export to
the Xen server so that Xen can take snapshots to that second exported
disk. If a problem does occur, then a restore is simple because then I
could do a dd from the snapshot lv (on the Xen server) to the other lv
(also on the Xen server).

But how about network traffic on this one? Everything I do will go
across iSCSI so it will go over the network.

And then I will manually have to greate lv-snapshots for each virtual
machine instead of just one big lv-snapshot as I could do when
lv_storage exists.

Any thoughts?

> > I can see the PV and LV's Xen created on lv_storage on the storage
> > server when I run pvs by the way...
> Yes, you will see them but you want the storage server's LVM to NOT use
> them under normal conditions.
> BTW, for backup, you may want snapshot-origin target rather than snapshot
> target directly..

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