On Fri, Apr 09, 2010 at 12:03:54PM -0400, Laine Stump wrote: > (Please forgive (and correct!) any inaccuracies in my description of > qemu's workings - I've only recently started looking at it directly, > rather than through the lens of libvirt) > > libvirt implements a "domain restore" operation by: > > 0) start with a previously saved domain image in a file > > 1) open the domain image, and connect it to a pipe > > 2) fork, connect the pipe to stdin, and exec qemu with "-incoming exec:cat" > > 3) execute "cont" in that qemu's monitor. > > (for those familiar with the code, you can look at the > src/qemu/eqmu_driver.c:qemudDomainRestore() in the libvirt source). > > Although this works successfully for most people, I'm consistently > seeing a problem on my particular hardware (Intel Core 2 Duo 2.2Ghz) > that causes this domain restore to fail. It seems that the "cont" > command takes effect before the restore is completed (possibly/probably > before it even starts?) resulting in a failed restore - the domain is > left in some random state, sometimes rebooting spontaneously, sometimes > just hung. > > If I insert a usleep(250 * 1000) between starting up qemu with > "-incoming exec:cat" and issuing "cont" to start the CPUs, the restore > is successful 100% of the time. > > I've been told that once the incoming migration starts, the monitor will > be non-responsive until it is complete. This should mean that as long as > the "cont" isn't issued until after the migration starts, it will be > blocked until the migration is complete, thus protecting us from the > race; for this reason (along with the fact that a 250msec sleep is > enough to cure the problem) I'm thinking it's likely the "cont" happens > before the migration starts. > > There is, of course, an "info migrate" command in the monitor that could > be used to assure the migration had completed before issuing "cont", but > that command only works for outgoing migrations, not incoming > (presumably if it was available, checking the info prior to the > migration starting would return "not started" (or something similar), > and once it had started, the entire monitor interface would block until > the migrate was completed).
Yep, I'd really like to see a 'info migrate' or equivalent that works for incoming migration, even if it can't give us progress info, just the status report is important to detect failure & completion. Daniel -- |: Red Hat, Engineering, London -o- http://people.redhat.com/berrange/ :| |: http://libvirt.org -o- http://virt-manager.org -o- http://deltacloud.org :| |: http://autobuild.org -o- http://search.cpan.org/~danberr/ :| |: GnuPG: 7D3B9505 -o- F3C9 553F A1DA 4AC2 5648 23C1 B3DF F742 7D3B 9505 :|