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

[Ian Campbell <Ian.Campbell@xxxxxxxxxx>]

On Thu, 2011-10-27 at 15:38 +0100, Alex Bligh wrote:
> 
> --On 27 October 2011 15:19:16 +0100 Stefano Stabellini 
> <stefano.stabellini@xxxxxxxxxxxxx> wrote:
> 
> >> 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.
> 
> I'm not sure I understand that. blkback on a loop device implies we
> can just use blkback on a real device. We are using a real device
> anyway in production (the file is just for testing).
> 
> So, should we be using tap:aio:/dev/... for a block device for
> speed?

tap:aio does not have a speed advantage -- in fact quite the opposite.
tap: gives you the flexibility to use non-raw block devices and
structured disk image types but is nothing like as fast as using a raw
block device with blkback.

The aio: part is an internal implementation detail of the tap: stuff
which should never have been exposed to the user.

Ian.

> 
> > 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.
> 
> I /think/ you mean only if we are using a file, so that shouldn't
> be relevant. Correct?
> 



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