Re: [Xen-devel] PV drivers on HVM using Xen 4.1.1

[Ian Campbell <Ian.Campbell@xxxxxxxxxx>]

On Thu, 2011-10-27 at 15:19 +0100, Stefano Stabellini wrote:
> On Thu, 27 Oct 2011, Alex Bligh wrote:
> > > you should use hda here because you want to
> > > make sure that an emulated IDE disk is created for you as well.
> > 
> > On Xen3.3 (and apparently on Xen 4) this happens anyway
> > 
> 
> Yes, but I wouldn't rely on that: if you specify xvda it means you want
> *only* a pv disk. However if you don't have any IDE disks configured,
> qemu realizes that you made a mistake in the config and setup one for
> you anyway.

Specifically _some_ qemu's realize...

> > > Also if /tmp/centos-pvd.img is a raw file, you might as well use file:
> > > rather than tap:aio.
> > 
> > There seems to be some doubt (see Ian's message) about whether this
> > changes the backend driver that is used. The final deployment application
> > is tap:aio with a block device, so that's why we're doing this.
>  
> If you are using XL, no matter if you specify tap:aio or file:, you are
> going to get qemu as disk backend if you are missing blktap.
> There is nothing wrong with that, except that qemu in 4.1 doesn't
> support linux aio so the performances are not very good. I am not sure
> which one is better: blkback on a loop device or qemu without linux aio,
> they are both rather slow.
> 
> In order to make it fast you can:
> 
> - use a dom0 kernel that provides blktap;
> 
> - use LVM with blkback;
> 
> - use upstream qemu with linux aio as device model and/or block backend.

Alex's real deployments use blkback on raw block devices so I think he
is ok.

Ian.



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