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



On Wed, Jul 03, 2013 at 11:54:15AM +0100, Richard W.M. Jones wrote:
> 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).

Yeah, for qcow2-inside-lvm to be practical you need to have a
continously active management application, to deal with ENOSPC
alerts. virt-manager doesn't satisfy that requirement.

> 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.

Agreed.

IIUC, the main reason why oVirt did the qcow2-inside-LVM think in the
first place was because they weren't happy with performance/functionality
of LVM snapshots. I know alot of work has been done to address this in
LVM since that time, so I'd just avoid qcow2-inside-lvm entirely.

Daniel
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