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On Thu, 27 Mar 2008 13:18:11 +0000, Hugo Mills <hugo@???> wrote:
> On Thu, Mar 27, 2008 at 12:03:17PM +0000, Tony Whitmore wrote:
>> On Thu, 27 Mar 2008 11:30:04 +0000, Andy Smith <andy@???> wrote:
>> > HVM is going to be quite slow compared to PV because of the emulated
>> > block and net drivers.
>>
>> Hmm, Hugo suggested the other way round. :/
>
> I probably didn't explain it very well, then. HVM will be faster
> executing code (particularly kernel code), but slower running I/O.
Ah right, that's much clearer.
> You
> can speed up the I/O by adding PV drivers. virtio is the Linux
> "native" PV implementation, and in theory is (or will be) supported by
> qemu, Xen, VMWare and lguest.
Virtio is pretty new to the kernel, only being in version 2.6.24 AFAICT. In
the case of a Debian Etch domU I guess I'd be looking for a backported
kernel if I want the virtio goodness? I assume there are then further
tweaks to be made inside the domU to get it to use the virtio drivers
rather than the 8139too or other default drivers.
Out of interest, do you know if the same applies to Windows domUs? Are
there equivalents of virtio drivers which make all the I/O nice and fast?
>> Unless I've misunderstood, surely the problem is
>> as much which NICs the hypervisor knows how to present to the domUs as
>> the drivers for the OS inside the domU itself?
>
> To some extent, yes.
>
> If you're talking paravirtual devices, the "NIC" presented by the
> host container is a very odd beastie, and doesn't really resemble any
> form of actual hardware. The PV driver then knows how to talk to that
> device, and the overall design of the device and its corresponding
> driver is aimed at maximal efficiency in data transfer.
Does that change with HVM? Presumably that emulates a "full" NIC, or is it
still an odd beastie?
Tony
