[Qemu-devel] Re: [kvm-devel] Feedback and errors

["Daniel P. Berrange" <berrange@redhat.com>]

On Fri, May 02, 2008 at 01:52:18PM +0300, Avi Kivity wrote:
> nadim khemir wrote:
> > Hi, great work.
> >
> > While playing with kvm-qemu I noticed a few points that might be of interrest:
> >
> > 1/ -loadvm and -snapshot don't work together. It works as if -loadvm wasn't 
> > passed as argument
> >
> > 2/ two instances of kvm can be passed the same -hda. There is no locking 
> > whatsoever. This messes up things seriously.

That depends entirely on what you are doing with the disk in the guest OS.

The disk could be hosting a cluster filesystem. The guest OS could be
running on a read-only root FS. The disk could be application raw data
storage which can be shared (eg Oracle RAC). 

And if the disk is backed by a physical volume which is accessible to
multiple hosts, locking on the VM's host won't provide any safety against
VMs on another host accessing it.

> These two are upstream qemu problems. Copying qemu-devel.
> 
> I guess using file locking by default would improve the situation, and 
> we can add a -drive ...,exclusive=no option for people playing with 
> cluster filesystems.

Turning on file locking by default will break existing apps / deployments
using shared disks. IMHO this is a policy decision that should be solved 
at ahigher level in the management stack where a whole world view is 
available rather than QEMU which only knows about its own VM & host.

Dan.
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