I switched back to virtio for the disk; it has the same issue. It apparently exits the VM via vp_notify(); in the trace I am looking at for almost 8 milliseconds. I had been thinking in terms of disk writes but of course this could be any disk activity so overlayroot will not solve this. It would seem that I might be looking at an inevitable disk rotation latency, which no host tuning will fix. Would a second CPU in the VM solve this or does virtio halt the whole VM for disk I/O? Or perhaps what I really need is to quiesce everything else in the VM to avoid all disk I/O? I suppose I could use a RAM root filesystem -- ugh. I find it a little puzzling that I have not seen disk I/O mentioned much in the realtime KVM literature I have found on the web. Joe Buehler
