[virt-tools-list] cache write back & barriers

[Richard W.M. Jones]

On Fri, Jun 14, 2013 at 12:53:04PM +0200, Stefan Hajnoczi wrote:
> On Thu, Jun 13, 2013 at 10:47:32AM +0200, folkert wrote:
> > Hi,
> > 
> > > > In virt-manager I saw that there's the option for cache writeback for
> > > > storage devices.
> > > > I'm wondering: does this also make kvm to ignore write barriers invoked
> > > > by the virtual machine?

Looking at current git, the cache types supported by virt-manager are:

 - none
 - writethrough
 - writeback
 - default [virt-manager only, not in virt-install]

These translate directly into the libvirt <driver ... cache="...">
field which you can find documented here:

http://libvirt.org/formatdomain.html#elementsDisks

As far as I can tell (from looking at libvirt sources) as long as you
have a modern qemu these will translate to the same names on the qemu
command line.

> > > No, that would be unsafe.  When the guest issues a flush then QEMU will
> > > ensure that data reaches the disk with -drive cache=writeback.
> > 
> > Aha so the writeback behaves like the consume harddisks with write-cache
> > on them.

In answer to the original question by 'folkert':

> > In that case maybe an extra note could be added to the virt-manager
> > (excellent software by the way!) that if the client vm supports
> > barriers, that write-back in that case then is safe. Agree?

I suspect the problem with doing this is it depends on the hypervisor.
Likely for qemu and Xen (since it uses a qemu device model) this would
be true.  Possibly not for other hypervisors that virt-manager can
control.

Generally speaking, it would be nice to document these properly and
also how they are implemented in different hypervisors, because I know
I for one don't find these settings very obvious.  So, patches welcome!

Rich.

-- 
Richard Jones, Virtualization Group, Red Hat http://people.redhat.com/~rjones
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