> -----Original Message-----
> From: xen-users-bounces@xxxxxxxxxxxxxxxxxxx
> [mailto:xen-users-bounces@xxxxxxxxxxxxxxxxxxx] On Behalf Of
> Mark Williamson
> Sent: 16 January 2007 17:07
> To: xen-users@xxxxxxxxxxxxxxxxxxx
> Cc: Tom Horsley; Goswin von Brederlow; James Rivera
> Subject: Re: [Xen-users] Getting better Disk IO
>
> > 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.
The reason the emulated IDE controller is quite slow is a consequence of
the emulation. The way it works is that the driver in the HVM domain
writes to the same IO ports that the real device would use. These writes
are intercepted by the hardware support in the processor and a VMEXIT is
issued to "exit the virtual machine" back into the hypervisor. The HV
looks at the "exit reason", and sees that it's an IO WRITE operation.
This operation is then encoded into a small packet and sent to QEMU.
QEMU processes this packet and responds back to HV to say "OK, done
that, you may continue". HV then does a VMRUN (or VMRESUME in the Intel
case) to continue the guest execution, which is probably another IO
instruction to write to the IDE controller. There's a total of 5-6 bytes
written to the IDE controller per transaction, and whilst it's possible
to combine some of these writes into a single write, it's not always
done that way. Once all writes for one transaction are completed, the
QEMU ide emulation code will perform the requested operation (such as
reading or writing a sector). When that is complete, a virtual interrupt
is issued to the guest, and the guest will see this as a "disk done"
interrupt, just like real hardware.
All these steps of IO intercepts takes several thousand cycles, which is
a bit longer than a regular IO write operation would take on the real
hardware, and the system will still need to issue the real IO operations
to perform the REAL hardware read/write corresponding to the virtual
disk (such as reading a file, LVM or physical partition) at some point,
so this is IN ADDITION to the time used by the hypervisor.
Unfortunately, the only possible improvement on this scenario is the
type "virtual-aware" driver that is described below.
[Using a slightly more efficient model than IDE may also help, but
that's going to be marginal compared to the benefits of using a
virtual-aware driver].
--
Mats
>
> 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!
>
> _______________________________________________
> Xen-users mailing list
> Xen-users@xxxxxxxxxxxxxxxxxxx
> http://lists.xensource.com/xen-users
>
>
>
_______________________________________________
Xen-users mailing list
Xen-users@xxxxxxxxxxxxxxxxxxx
http://lists.xensource.com/xen-users
|
Home