On Wed, Oct 08, 2008 at 12:03:33PM +0100, Daniel P. Berrange wrote:
> QEMU defaults to allowing the host OS to cache all disk I/O. THis has a
> couple of problems
>
> - It is a waste of memory because the guest already caches I/O ops
> - It is unsafe on host OS crash - all unflushed guest I/O will be
> lost, and there's no ordering guarentees, so metadata updates could
> be flushe to disk, while the journal updates were not. Say goodbye
> to your filesystem.
> - It makes benchmarking more or less impossible / worthless because
> what the benchmark things are disk writes just sit around in memory
> so guest disk performance appears to exceed host diskperformance.
>
> This patch disables caching on all QEMU guests. NB, Xen has long done this
> for both PV & HVM guests - QEMU only gained this ability when -drive was
> introduced, and sadly kept the default to unsafe cache=on settings.
Right !
I think for integrity reason we should revert that default at the
libvirt level and swicth caching to off. I would not be against a
way to reactivate it optionally, assuming we have a clean way to express
it at the XML level (I don't think we have currently maybe an
optional cache="on|off" attribute could be added to device/disk/target)
because in some circumstances like cache of read-only devices available
to multiple domain, it can make sense to keep caching on the host os.
So I'm fine with the patch going in as-is, but maybe we need
one patch on top to reenable the cache in an explicit case by case
basis.
Daniel
P.S.: can you try to get patches with -p to get the contextual function
without them it's harder to review exactly where things goes,
especially when there is a line number shift due to other pending
patches, thanks !
--
Daniel Veillard | libxml Gnome XML XSLT toolkit http://xmlsoft.org/
daniel@xxxxxxxxxxxx | Rpmfind RPM search engine http://rpmfind.net/
http://veillard.com/ | virtualization library http://libvirt.org/
--
Libvir-list mailing list
Libvir-list@xxxxxxxxxx
https://www.redhat.com/mailman/listinfo/libvir-list