Re: [Xen-users] Paravirtualized vs HVM - how big is the performance gap?

["Fajar A. Nugraha" <fajar@xxxxxxxxx>]

On Fri, Mar 27, 2009 at 4:08 PM, Tim Edwards <tedwards@xxxxxxxxx> wrote:
> I see 3 possibilities but don't really know if they are realistic so
> need people's thoughts on them:
>
> * Find a way to install SL4.x in a HVM domU and then convert that domU
> to paravirt mode (I'm trying this now but with no success)
>

If you have problems converting, try my method
http://lists.xensource.com/archives/html/xen-users/2009-03/msg00372.html

It needs /boot and "/" to be on the same partition (e.g. hda1) on HVM
install, without LVM, to simplify things. I haven't tested it for SL 4
(don't have an OS template handy atm), but it should work with the
differences that the kernel package is kernel-xenU instead of
kernel-xen, and the source tree is HVM installation instead of OS
template, and hda1 instead of sda1.
Let me know if you experience problems.

> * There is some technique to allow booting of paravirt VMs off of .iso
> images or over PXE (or any other method that could lead to a Kickstart
> installation)?

Try http://wiki.centos.org/HowTos/Xen/InstallingCentOSDomU
domU installation kernel and initrd is from
http://mirror.centos.org/centos/4/os/i386/images/xen/
kickstart installation URL is http://mirror.centos.org/centos/4/os/i386/

>
> * Just run 4.x in HVM mode - roughly how big is the performance impact
> and how could I minimise it?

Should be pretty big. I haven't tested HVM Linux specifically (never
needed one), but you can look at my numbers for Windows HVM on
opensolaris dom0:

Disk peformance, command line : dd if=/dev/zero of=test2G bs=1M count=2048
without GPLPV : 6.6 MB/s
with GPLPV : 12.1 MB/s
Linux PV : 24.6 MB/s

Network performance, with iperf:
Without GPLPV : dom0->domU 38.8 Mbps, domU->dom0 24.4 Mbps,
With GPLPV : dom0->domU 148 Mbps, domU->dom0 446 Mbps
Linux PV : dom0->domU 556 Mbps, domU->dom0 631Mbps

Regards,

Fajar

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