On Mon, Aug 29, 2016 at 06:56:42PM +0000, Felipe Franciosi wrote: > Heya! > > > On 29 Aug 2016, at 08:06, Stefan Hajnoczi <stefa...@gmail.com> wrote: > > > > At KVM Forum an interesting idea was proposed to avoid > > bdrv_drain_all() during live migration. Mike Cui and Felipe Franciosi > > mentioned running at queue depth 1. It needs more thought to make it > > workable but I want to capture it here for discussion and to archive > > it. > > > > bdrv_drain_all() is synchronous and can cause VM downtime if I/O > > requests hang. We should find a better way of quiescing I/O that is > > not synchronous. Up until now I thought we should simply add a > > timeout to bdrv_drain_all() so it can at least fail (and live > > migration would fail) if I/O is stuck instead of hanging the VM. But > > the following approach is also interesting... > > > > During the iteration phase of live migration we could limit the queue > > depth so points with no I/O requests in-flight are identified. At > > these points the migration algorithm has the opportunity to move to > > the next phase without requiring bdrv_drain_all() since no requests > > are pending. > > I actually think that this "io quiesced state" is highly unlikely to _just_ > happen on a busy guest. The main idea behind running at QD1 is to naturally > throttle the guest and make it easier to "force quiesce" the VQs. > > In other words, if the guest is busy and we run at QD1, I would expect the > rings to be quite full of pending (ie. unprocessed) requests. At the same > time, I would expect that a call to bdrv_drain_all() (as part of > do_vm_stop()) should complete much quicker. > > Nevertheless, you mentioned that this is still problematic as that single > outstanding IO could block, leaving the VM paused for longer. > > My suggestion is therefore that we leave the vCPUs running, but stop picking > up requests from the VQs. Provided nothing blocks, you should reach the "io > quiesced state" fairly quickly. If you don't, then the VM is at least still > running (despite seeing no progress on its VQs). > > Thoughts on that?
If the guest experiences a hung disk it may enter error recovery. QEMU should avoid this so the guest doesn't remount file systems read-only. This can be solved by only quiescing the disk for, say, 30 seconds at a time. If we don't reach a point where live migration can proceed during those 30 seconds then the disk will service requests again temporarily to avoid upsetting the guest. I wonder if Juan or David have any thoughts from the live migration perspective? Stefan
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