Re: [Xen-users] qcow2 and Xen

[Paul Durrant <Paul.Durrant@xxxxxxxxxx>]

> -----Original Message-----
> From: George Dunlap <dunlapg@xxxxxxxxx>
> Sent: 15 July 2019 16:36
> To: Stefan Malte Schumacher <stefan.schumacher.netfed@xxxxxxxxx>
> Cc: xen-users <xen-users@xxxxxxxxxxxxxxxxxxxx>; Paul Durrant 
> <Paul.Durrant@xxxxxxxxxx>
> Subject: Re: [Xen-users] qcow2 and Xen
> 
> On Mon, Jul 15, 2019 at 3:07 PM Stefan Malte Schumacher
> <stefan.schumacher.netfed@xxxxxxxxx> wrote:
> >
> > Hello,
> >
> > we are currently planning to create about 40 new VMs on Debian 10 and there 
> > are
> > some technology decisions we want to make before starting. We have already 
> > decided
> > that we will use PVH instead of PV guests. Now we are debating how to store 
> > the VMs.
> > Until now we have used the traditional approach with a LVM. A colleague 
> > showed me some of the things
> he can do with proxmox and qcow2 and I would like to give qcow2-images a try. 
> We have midterm plans of
> migrating to a SAN and this would most likely be easier with images instead 
> of a LVM.
> >
> > I have found out on my own that Xen supports qcow2 in theory, but I still 
> > have two questions:
> >
> > 1) How mature is Xen's support for qcow2 as compared to LVM or raw images?
> 
> For qcow2, Xen uses a mode called `qdev`, where an instance of QEMU
> itself acts as a disk backend; this means that there should be
> absolutely no issues with image compatibility, since it's exactly the
> same code reading the image as in upstream QEMU.  Functionally, it
> gets tested regularly and so should have no stability issues.
> 
> I'm not sure if anyone has been investing in performance -- perhaps
> Paul can comment on that.

I and others have added some performance improvements (e.g. multi-page ring and 
iothread) but there are more in the pipeline (grant op batching for one) when 
time permits. Citrix Hypervisor is now actively using QEMU and qcow2 images so 
they do get a reasonable level scrutiny in a Xen environment.

  Paul
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