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



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
http://wpkg.org


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