On Mon, Jul 30, 2012 at 11:25:51AM +0930, Rusty Russell wrote:

I consider this approach a half-way step.  Quick attempts on my laptop
and I couldn't find a case where the bio path was a loss, but in theory
if the host wasn't doing any reordering and it was a slow device, you'd
want the guest to do so.

I'm not sure if current qemu can be configured to do such a thing?

The host kernel will do the I/O scheduling for you unless you explicitly
disable it.  And we should be able to assume an administrator will only
disable it when they have a reason for it - if not they'll get worse
performance for non-virtualized workloads as well.