Re: [Qemu-devel] [PATCH] migration: flush the bdrv before stopping VM

[Kevin Wolf <kwolf@redhat.com>]

Am 25.03.2015 um 11:50 hat Juan Quintela geschrieben:
> "Li, Liang Z" <liang.z.li@intel.com> wrote:
> >> >> > Right now, we don't have an interface to detect that cases and got
> >> >> > back to the iterative stage.
> >> >>
> >> >> How about go back to the iterative stage when detect that the
> >> >> pending_size is larger Than max_size, like this:
> >> >>
> >> >> +                /* do flush here is aimed to shorten the VM downtime,
> >> >> +                 * bdrv_flush_all is a time consuming operation
> >> >> +                 * when the guest has done some file writing */
> >> >> +                bdrv_flush_all();
> >> >> +                pending_size = qemu_savevm_state_pending(s->file, max_size);
> >> >> +                if (pending_size && pending_size >= max_size) {
> >> >> +                    qemu_mutex_unlock_iothread();
> >> >> +                    continue;
> >> >> +                }
> >> >>                   ret = vm_stop_force_state(RUN_STATE_FINISH_MIGRATE);
> >> >>                   if (ret >= 0) {
> >> >>                       qemu_file_set_rate_limit(s->file, INT64_MAX);
> >> >>
> >> >> and this is quite simple.
> >> >
> >> > Yes, but it is too simple. If you hold all the locks during
> >> > bdrv_flush_all(), your VM will effectively stop as soon as it performs
> >> > the next I/O access, so you don't win much. And you still don't have a
> >> > timeout for cases where the flush takes really long.
> >> 
> >> This is probably better than what we had now (basically we are "meassuring"
> >> after bdrv_flush_all how much the amount of dirty memory has changed,
> >> and return to iterative stage if it took too much.  A timeout would be better
> >> anyways.  And an interface te start the synchronization sooner
> >> asynchronously would be also good.
> >> 
> >> Notice that my understanding is that any proper fix for this is 2.4 material.
> >
> > Then, how to deal with this issue in 2.3, leave it here? or make an
> > incomplete fix like I do above?
> 
> I think it is better to leave it here for 2.3. With a patch like this
> one, we improve in one load and we got worse in a different load (depens
> a lot in the ratio of dirtying memory vs disk).  I have no data which
> load is more common, so I prefer to be conservative so late in the
> cycle.  What do you think?

I agree, it's too late in the release cycle for such a change.

Kevin