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

Tomasz Chmielewski mangoo at wpkg.org
Tue Nov 20 13:06:47 UTC 2007

S. J. van Harmelen schrieb:
> Thanks for your reaction!
> On Tue, 2007-11-20 at 12:37 +0100, Tomasz Chmielewski wrote:
>> S. J. van Harmelen schrieb:
>>> Hi list,
>>> In advance my excusses for this radar long post (although it's easy
>>> readable ;), but I want to make sure that I understand it correctly so I
>>> don't end up making a very costly mistake.
>>> I have a storage server (Debian Etch) with mutlipath-tools running and
>>> on top of that I use IET iscsi-target software to export the multipathed
>>> device to a XenSource server.
>>> XenSource creates a PV on the entire exported disk, and then creates a
>>> few LV's when I create some virtual machines.
>> Which IMO is a pity, as logically, LVM exists and is usable on that 
>> given Xen server only. This means you can't really use multiple Xen 
>> servers, live migration etc.
> Are you sure about that? Accoring to Xen lvm over iSCSI is considered as
> shared storage that can be used for live migration
> (http://docs.xensource.com/XenServer/4.0.1/installation/ch03s03.html):
> <snip>
> 3.3.3. XenServer Hosts with shared iSCSI storage

This implies turning a Xen server into a NAS server, as I understand?
Yes, this should work - I have a custom Xen setup, and I don't connect a 
NAS and a Xen server on one host.
But according to the description, yes, it should work.


>>> Questions that I think of then are if it's not a problem that XenSource
>>> then creates a new PV and some LV's in je LV I created adn exported on
>>> the storage server. Is that a problem, or should this work fine?
>> Hmm? I don't think I understand what you mean.
> I mean I will have 2 sets of PV's and LV's. One set on the storage
> server (that has one PV and one LV that both span the whole disk), and
> one set op the Xen server (the ones that Xen makes by itself when I add
> a new vitrual machine).
> Now the one LV from the storage server that Xen sees is being exported
> true iSCSI, so it wouldn't know that it is infact a LV that it's talking
> to. As far as the Xen server conserns this is just a raw disk. So it
> will then just create the needed PV and LV's on it to provision virtual
> mahines.
> The question was if this PV/LV in another PV/LV (on another physical
> machine) can do any harm?

Should work, as long as you don't use it on two different machines 
(i.e., target/initiator) as LVM at the same time.

>>> And another question is how I can then restore a single LV Xen created,
>>> from the snapshot of the LV that spans the whole disk on the storage
>>> server? In that case I can not just revert to the old disk before taking
>>> the snapshot, because then all the LV's created by Xen will be set back
>>> to that point, and not just the LV that went bad.
>> # Will only work if snapshot size is equal or greater than
>> # the original volume
>> dd if=/dev/LVM/volume-snapshot of=/dev/LVM/volume
>> # or, if the allowed snapshot size is smaller, we don't want our
>> # precious snapshot dropped
>> dd if=/dev/LVM/volume-snapshot of=/dev/LVM/new-volume
>> dd if=/dev/LVM/new-volume of=/dev/LVM/volume
> Oke, but how about this when using LVM as I just described a few lines
> above here with the PV/LV in another PV/LV setup. How can I then restore
> a snapshot of a virtual machines' LV from the snapshot of the LV on the
> storage server?

As a prerequisite, you need to "see" that LVM.
But could you be more specific on what you want (write a bit more 
details in points, write a small diagram etc.)?

Tomasz Chmielewski

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