On Thu, Dec 26, 2019 at 05:40:22PM +0800, 张海斌 wrote: > Stefan Hajnoczi <stefa...@redhat.com> 于2019年3月29日周五 上午1:08写道： > > > > On Thu, Mar 28, 2019 at 05:53:34PM +0800, 张海斌 wrote: > > > hi, stefan > > > > > > I have faced the same problem you wrote in > > > https://lists.gnu.org/archive/html/qemu-devel/2016-08/msg04025.html > > > > > > Reproduce as follow: > > > 1. Clone qemu code from https://git.qemu.org/git/qemu.git, add some > > > debug information and compile > > > 2. Start a new VM > > > 3. In VM, use fio randwrite to add pressure for disk > > > 4. Live migrate > > > > > > Log show as follow: > > > [2019-03-28 15:10:40.206] /data/qemu/cpus.c:1086: enter do_vm_stop > > > [2019-03-28 15:10:40.212] /data/qemu/cpus.c:1097: call bdrv_drain_all > > > [2019-03-28 15:10:40.989] /data/qemu/cpus.c:1099: call > > > replay_disable_events > > > [2019-03-28 15:10:40.989] /data/qemu/cpus.c:1101: call bdrv_flush_all > > > [2019-03-28 15:10:41.004] /data/qemu/cpus.c:1104: done do_vm_stop > > > > > > Calling bdrv_drain_all() costs 792 mini-seconds. > > > I just add a bdrv_drain_all() at start of do_vm_stop() before > > > pause_all_vcpus(), but it doesn't work. > > > Is there any way to improve live-migration downtime cause by > > > bdrv_drain_all()?
I believe there were ideas about throttling storage controller devices during the later phases of live migration to reduce the number of pending I/Os. In other words, if QEMU's virtio-blk/scsi emulation code reduces the queue depth as live migration nears the handover point, bdrv_drain_all() should become cheaper because fewer I/O requests will be in-flight. A simple solution would reduce the queue depth during live migration (e.g. queue depth 1). A smart solution would look at I/O request latency to decide what queue depth is acceptable. For example, if requests are taking 4 ms to complete then we might allow 2 or 3 requests to achieve a ~10 ms bdrv_drain_all() downtime target. As far as I know this has not been implemented. Do you want to try implementing this? Stefan
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