Re: XP installation speed: KVM vs Xen

[Anthony Liguori <anthony-rdkfGonbjUSkNkDKm+mE6A@public.gmane.org>]

Andrew Olney wrote:
> I've been migrating from Xen to KVM for a wide variety of reasons.
>
> I've noticed, however, that XP installs much much faster with Xen (3.03
> xen-image-xen0-2.6.17-6-generic-xen0 ubuntu) than with KVM (14).
>
> I haven't clocked it, but for KVM it takes longer than realtime (by the
> XP install clock) and with Xen it takes less than realtime.
>
> I assume that this is all file I/O related, which raises the question:
> what file system setup optimizes KVM I/O, e.g. qcow vs. raw?
>   

Xen should outperform KVM wrt disk performance.  Xen is based on QEMU 
0.8.2 and has a ton of patches including one that aggressive reports IO 
completions to the host asynchronously.

KVM uses a version of QEMU that is significantly closer to mainline QEMU 
(almost no changes that aren't KVM related).  Eventually, all of the KVM 
changes should make it into mainline QEMU whereas that probably will 
never happen with Xen.

QEMU only got asynchronous IO support in 0.9.0.  Unfortunately, the 
implementation actually results in a regression in performance.  Some 
hacks can fix it though.

Of course, the SCSI emulation in 0.9.0 improves performance by quite a 
lot (to the point where it's better than Xen).  However, some work is 
needed to make SCSI disks bootable and to expose SCSI command line options.

Regards,

Anthony Liguori

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