Re: [Qemu-discuss] Disk Caching when domain is suspended.

[Matthew Schumacher]

On 11/14/18 11:50 AM, Matthew Schumacher wrote:
> Qemu devs,
>
> I have a process where I suspend a domain, then migrate some of it's 
> disk elsewhere, then live migrate (to copy cpu state and memory) the 
> domain to another host and resume.  The migration is marked unsafe in 
> libvirt because disk caching is enabled even though the domain is in 
> the suspend state.
>
> Can someone confirm that the disk cache is flushed as part of the 
> suspend process?  If not, it seems reasonable that you would want the 
> disk completely quiesced while the domain was in the suspend state so 
> that you can perform tasks without worry that the disk is still changing.
>
> Thanks,
> schu
I found this code in cpus.c

static int do_vm_stop(RunState state, bool send_stop)
{
    int ret = 0;

    if (runstate_is_running()) {
        cpu_disable_ticks();
        pause_all_vcpus();
        runstate_set(state);
        vm_state_notify(0, state);
        if (send_stop) {
            qapi_event_send_stop(&error_abort);
        }
    }

    bdrv_drain_all();
    replay_disable_events();
    ret = bdrv_flush_all();

    return ret;
}

That would lead me to believe that it does flush the cache with 
bdrv_drain_all and bdrv_flush_all, but I'm not entirely sure without a 
bit of code analysis, so a quick confirmation would be welcome.

Thanks,
schu