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Re: [virt-tools-list] Virt-Manager, diskformat on LVM

On Tue, Jul 02, 2013 at 12:03:10PM +0200, Marko Weber | ZBF wrote:
> is this a restriction by virt-manager? is it best choose to use
> "raw" on lvm?  in many qemu-kvm books i read best way will be to use
> qcow2.

Despite what you may have read, I don't think using qcow2 on top of
LVM is a good idea unless you really know what you're doing.  qcow2
files generally grow as they are being used up to a maximum size which
is not well-defined.  LVs of course are fixed size, and can be grown
only using an explicit command (lvresize).

On oVirt/RHEV the LVs are resized on demand.  This is done by having
qemu pause when it detects that it has run out of space during a qcow2
write.  This fact is signalled up to the oVirt management layer which
invokes lvresize and restarts the guest.  libvirt handles the signals
but the resizing happens in the management layer above libvirt.  There
is no such "layer above libvirt" when using virt-manager (except
perhaps virt-manager itself).

The only alternative would be to over-provision the LV so it can cope
with the largest possible qcow2 file.  But then you might as well
using raw which will faster and easier to reason about.


Richard Jones, Virtualization Group, Red Hat http://people.redhat.com/~rjones
virt-top is 'top' for virtual machines.  Tiny program with many
powerful monitoring features, net stats, disk stats, logging, etc.

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