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Re: [Xen-users] Exploring Xen 3.3.2 with LVM, iSCSI and PVSCSI...

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Subject: Re: [Xen-users] Exploring Xen 3.3.2 with LVM, iSCSI and PVSCSI...
From: Simon Hobson <linux@xxxxxxxxxxxxxxxx>
Date: Fri, 7 Aug 2009 09:04:34 +0100
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Thiago Camargo Martins Cordeiro wrote:

I'm trying to explore the possibilities of PVSCSI, as for example, online resize, look:

At the Debian Lenny dom0 with Xen 3.3.2 and Linux 2.6.18 compiled from xen.org:

1) lvcreate -L 10G -n guest01-root HyperVG01;
2) configure the guest01 PV domU with PVSCSI using HyperVG01/guest01-root as partition, not as virtual disk;
3) start guest01;
4) lvextend -L +15G HyperVG01/guest01-root.

Should I need to reboot the domU to be able to use this +15G? Or the resize is online?!

As far as I know, the new size will not get passed to the client. A reboot is not sufficient, I think you need to shutdown and recreate the client. There are xm commands to dettach and attach storage volumes from the guest, so if you are able to unmount the volume in the guest then this may be a means of doing a resize without a reboot - I haven't used these commands.

If yes, the resize can be done online, what happens if I put the iSCSI target/client between the LVM and PVSCSI, the online resize will still be available?

You mean, share the volume form Dom0 with iSCSI and mount it in the guest as an iSCSI volume ? I think you'll find performance is poor. I've tried mounting an iSCSI volume in a guest (from an external device, not Dom0) and performance was very poor - I believe due to the single thread nature of Dom0 networking. I suspect the same issue would apply if the volume was being exported from Dom0.

In my case, I mounted the iSCSI volume in Dom0 and passed it to the guest as a normal physical volume. Not my preferred option as it makes migration harder, but performance is good.

But with iSCSI, you can do an online resize, but you still need to unmount the filesystem in the guest before you can resize it.

Simon Hobson

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