> I've been hoping to see replies to this, but lacking good information
> here is the state of my confusion on virtual machine disks:
>
> If you read the docs for configuring disks on domu and hvm machines,
> you'll find a gazillion or so ways to present the disks to the virtual
> machine.
There are quite a lot of options, it's true ;-)
> One of those ways (who's name I forget) provides (if I understand things,
> which I doubt :-), provides a special kind of disk emulation designed to
> be driven by special drivers on the virtual machine side. The combination
> gives near direct disk access speeds in the virtual machine.
>
> The catch is that you need those drives for the kernel on the virtual
> machine side. They may already exist, you may have to build them, and
> depending on the kernel version, they may be hard to build.
>
> Perhaps someone who actually understands this could elaborate?
Basically yes, that's all correct.
To summarise:
PV guests (that's paravirtualised, or Xen-native) use a Xen-aware block device
that's optimised for good performance on Xen.
HVM guests (Hardware Virtual Machine, fully virtualised and unaware of Xen)
use an emulated IDE block device, provided by Xen (actually, it's provided by
the qemu-based device models, running in dom0).
The HVM emulated block device is not as optimised (nor does it lend itself to
such effective optimisation) for high virtualised performance as the
Xen-aware device. Therefore a second option is available for HVM guests: an
implementation of the PV guest device driver that is able to "see through"
the emulated hardware (in a secure and controlled way) and talked directly as
a Xen-aware block device. This can potentially give very good performance.
I don't know if these drivers are included in any Linux distributions yet, but
they are available in the Xen source tree so that you can build your own, in
principle. Windows versions of the drivers are included in XenSource's
products, I believe - including the free (as in beer) XenExpress platform.
There are potentially other options being developed, including an emulated
SCSI device that should improve the potential for higher performance IO
emulation without Xen-aware drivers.
Hope that clarifies things!
Cheers,
Mark
--
Dave: Just a question. What use is a unicyle with no seat? And no pedals!
Mark: To answer a question with a question: What use is a skateboard?
Dave: Skateboards have wheels.
Mark: My wheel has a wheel!
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