Re: [Qemu-devel] [RFC] Disk integrity in QEMU

[Jamie Lokier <jamie@shareable.org>]

Rik van Riel wrote:
> >If there are multiple qemu/kvm guests accessing the same disk image in
> >a cluster, provided the host cluster filesystem uses a fully coherent
> >protocol, ordinary cached reads should be fine.  (E.g. not NFS).
> 
> The problem is when the synchronization only happens in the guests,
> which is a legitimate and common configuration.
> 
> Ie. the hosts just pass through the IO and the guests run a GFS
> cluster.

Ok, if you are using multiple hosts with a non-coherent host
filesystem for the virtual disk, or a non-coherent host block device
for the virtual disk, it won't work.

But why would you do that?

What is the legitimate and common configuration where you'd share a
virtual among multiple _hosts_ with a non-coherent host file/device
sharing protocol and expect it to work?

Do you envisage qemu/kvm using O_DIRECT over NFS or SMB on the host, or
something like that?

> Caching either reads or writes at the host level causes problems.

But only if the hosts are using a non-coherent protocol.  Not having a
visible effect (except timing) is pretty much the definition of
coherent caching.

Is there a reason why you wouldn't use, say, GFS on the host (because
it claims to be coherent)?  Does performance suck relative to O_DIRECT
over NFS?

-- Jamie