[libvirt-users] changing to cache='none' on the fly?

[Daniel P. Berrange]

On Mon, Jan 09, 2017 at 10:29:01AM -0800, W Kern wrote:
> As is already well documented, we find that we need cache='none' to support
> migration, otherwise there is the chance of a hang and/or failure to pivot.
> 
> However we prefer the default of cache=writethrough when operating in
> production.
> 
> Our practice is to 'shutdown' the VM completely make the change with virsh
> edit,  then restart.  Then we have to repeat the process to revert back once
> we migrate.
> 
> Is it possible to change that function on the fly and avoid the
> shutdown/start process?

Nope, there's no way to change the cache mode on the fly.

> Note: we have had inconsistent results with virsh edit first and then a
> reboot. A complete shutdown seems to be necessary, but I am hoping there is
> some other procedure available.

"virsh edit" will always just change the offline persistent config.

This config will be used next time the guest is *cold* booted. If
you just do a reboot inside the guest, this is a warm reboot, and
so the QEMU process never stops running. This means we don't pick
up the new offline config you wrote.

Regards,
Daniel
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