[libvirt] Would it make sense to have a <disk device='logicalvolume'>?

[Daniel P. Berrange]

On Tue, Jan 10, 2012 at 12:00:27PM +0100, Francesco Pretto wrote:
> The questions is this:
> 1) would it make sense to expose a block device in the host to be
> mapped like a "logical volume" in the guest (I mean: not actually
> exposing it as a physical disk)?
> 2) Is there any existing hypervisor that could be made to work like this?

The problem is that a logical volume is a software concept in the
host OS, and a software concept in the guest OS. Block devices
passed to guests are all types of disk hardware, whether IDE,
SCSI, USB, Virtio Block (a sort of paravirt SCSI).

To do what you describe, you'd need to write a virtio-lvm paravirt
driver for Linux & QEMU. And even then, I'm not sure the guest
OS LVM tools will like seeing a logical volume, without any
corresponding volume group, or physical volumes visible. Indeed
if the guest doesn't have any VG or PVs visible, then you loose
the most important benefit of having a logical volume - the ability
to resize it. So it all seems rather pointless to me.

Regards,
Daniel
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