* Daniel P. Berrangé ([email protected]) wrote: > On Fri, Jul 03, 2020 at 06:00:50PM +0100, Dr. David Alan Gilbert wrote: > > * Daniel P. Berrangé ([email protected]) wrote: > > > On Fri, Jul 03, 2020 at 05:22:46PM +0100, Dr. David Alan Gilbert wrote: > > > > * Daniel P. BerrangÃÆé ([email protected]) wrote: > > > > > On Fri, Jul 03, 2020 at 05:10:12PM +0100, Dr. David Alan Gilbert > > > > > wrote: > > > > > > * Daniel P. BerrangÃÆé ([email protected]) wrote: > > > > > > > On Fri, Jul 03, 2020 at 04:49:33PM +0100, Dr. David Alan Gilbert > > > > > > > wrote: > > > > > > > > * Daniel P. BerrangÃÆé ([email protected]) wrote: > > > > > > > > > On Thu, Jul 02, 2020 at 01:12:52PM -0500, Eric Blake wrote: > > > > > > > > > > On 7/2/20 12:57 PM, Daniel P. BerrangÃÆé wrote: > > > > > > > > > > > savevm, loadvm and delvm are some of the few commands > > > > > > > > > > > that have never > > > > > > > > > > > been converted to use QMP. The primary reason for this > > > > > > > > > > > lack of > > > > > > > > > > > conversion is that they block execution of the thread for > > > > > > > > > > > as long as > > > > > > > > > > > they run. > > > > > > > > > > > > > > > > > > > > > > Despite this downside, however, libvirt and applications > > > > > > > > > > > using libvirt > > > > > > > > > > > has used these commands for as long as QMP has existed, > > > > > > > > > > > via the > > > > > > > > > > > "human-monitor-command" passthrough command. IOW, while > > > > > > > > > > > it is clearly > > > > > > > > > > > desirable to be able to fix the blocking problem, this is > > > > > > > > > > > not an > > > > > > > > > > > immediate obstacle to real world usage. > > > > > > > > > > > > > > > > > > > > > > Meanwhile there is a need for other features which > > > > > > > > > > > involve adding new > > > > > > > > > > > parameters to the commands. This is possible with HMP > > > > > > > > > > > passthrough, but > > > > > > > > > > > it provides no reliable way for apps to introspect > > > > > > > > > > > features, so using > > > > > > > > > > > QAPI modelling is highly desirable. > > > > > > > > > > > > > > > > > > > > > > This patch thus introduces trival savevm, loadvm, delvm > > > > > > > > > > > commands > > > > > > > > > > > > > > > > > > > > trivial > > > > > > > > > > > > > > > > > > > > > to QMP that are functionally identical to the HMP > > > > > > > > > > > counterpart, including > > > > > > > > > > > the blocking problem. > > > > > > > > > > > > > > > > > > > > Should we name them 'x-savevm', 'x-loadvm', 'x-delvm' to > > > > > > > > > > give ourselves room > > > > > > > > > > to change them when we DO solve the blocking issue? Or > > > > > > > > > > will the solution of > > > > > > > > > > the blocking issue introduce new QMP commands, at which > > > > > > > > > > point we can add QMP > > > > > > > > > > deprecation markers on these commands to eventually retire > > > > > > > > > > them? > > > > > > > > > > > > > > > > > > I was in two minds about this, so I'm open to arguments > > > > > > > > > either way. > > > > > > > > > > > > > > > > > > The primary goal is for libvirt to consume the APIs as soon > > > > > > > > > as possible, > > > > > > > > > and generally libvirt doesn't want todo this is they are > > > > > > > > > declared experimental > > > > > > > > > via a "x-" prefix. So that pushes me away from "x-". > > > > > > > > > > > > > > > > > > If we don't have an "x-" prefix and want to make changes, we > > > > > > > > > can add extra > > > > > > > > > parameters to trigger new behaviour in backwards compatible > > > > > > > > > manner. Or we can > > > > > > > > > simply deprecate these commands, deleting them 2 releases > > > > > > > > > later, while adding > > > > > > > > > completely new commands. > > > > > > > > > > > > > > > > > > If we think the prposed design will definitely need > > > > > > > > > incompatible changes in > > > > > > > > > a very short time frame though, that would push towards "x-". > > > > > > > > > > > > > > > > > > So IMHO the right answer largely depends on whether there is > > > > > > > > > a credible > > > > > > > > > strategy to implement the ideal non-blocking solution in a > > > > > > > > > reasonable amount > > > > > > > > > of time. I can't justify spending much time on this myself, > > > > > > > > > but I'm willing > > > > > > > > > to consider & try proposals for solving the blocking problem > > > > > > > > > if they're not > > > > > > > > > too complex / invasive. > > > > > > > > > > > > > > > > Remind me, what was the problem with just making a block: > > > > > > > > migration > > > > > > > > channel, and then we can migrate to it? > > > > > > > > > > > > > > migration only does vmstate, not disks. The current blockdev > > > > > > > commands > > > > > > > are all related to external snapshots, nothing for internal > > > > > > > snapshots > > > > > > > AFAIK. So we still need commands to load/save internal snapshots > > > > > > > of > > > > > > > the disk data in the qcow2 files. > > > > > > > > > > > > > > So we could look at loadvm/savevm conceptually as a syntax sugar > > > > > > > above > > > > > > > a migration transport that targets disk images, and blockdev QMP > > > > > > > command > > > > > > > that can do internal snapshots. Neither of these exist though and > > > > > > > feel > > > > > > > like a significantly larger amount of work than using existing > > > > > > > functionality > > > > > > > that is currently working. > > > > > > > > > > > > I think that's what we should aim for; adding this wrapper isn't > > > > > > gaining > > > > > > that much without moving a bit towards that; so I would stick with > > > > > > the > > > > > > x- for now. > > > > > > > > > > The question is how much work that approach will be and whether > > > > > anyone can > > > > > realistically commit to doing that ? It looks like a much larger > > > > > piece of > > > > > work in both QEMU and libvirt side to do that. I don't want to see us > > > > > stuck > > > > > with a x-savevm for years because no one has resource to work on the > > > > > perfect > > > > > solution. If we did get a perfect solution at a point in future, we > > > > > can > > > > > still deprecate and then remove any "savevm" command we add to QMP. > > > > > > > > I'd at least like to understand that we've got a worklist for it though. > > > > We've already got qemu_fopen_bdrv - what's actually wrong with that - is > > > > that enough to do the migrate to a stream (given a tiny amount of > > > > syntax) ? > > > > > > It is close. The migration code works with the QEMUFile layer, but in > > > terms > > > of the monitor commands the current framework expects a QIOChannel based > > > QEMUFile. It would be possible to add new helpers to work with the bdrv > > > backed QEMUFile. The ideal would be to create a QIOChannel impl that is > > > backed by a block device though. At that point there would only be a > > > single > > > QEMUFile impl based on QIOChannel. > > > > > > That would be another step closer to unlocking the ability to eliminate > > > the > > > QEMUFile wrapper entirely. QEMUFile does I/O buffering too for > > > performance, > > > that could be done in a QIOChannel layer too, as that's a concept useful > > > for other QIOChannel users too. > > > > There's some separate patches on list to do buffering in bdrv for > > vmsave because apparently the qemufile ones don't play well with it. > > > > > That's still only the vmstate part though, and a solution is needed for > > > the internal snapshot handling. > > > > Which bit is the bit that makes it block? > > The entire save_snapshot / load_snapshot methods really. They doing I/O > operations throughout and this executes in context of the thread running > the monitor, so their execution time is proportional to I/O time.
Is there any reason for the migrationy bit to run synchronously then? Could those snapshotting things be done with any of the block copy processes that run asynchronously? Dave > > Regards, > Daniel > -- > |: https://berrange.com -o- https://www.flickr.com/photos/dberrange :| > |: https://libvirt.org -o- https://fstop138.berrange.com :| > |: https://entangle-photo.org -o- https://www.instagram.com/dberrange :| -- Dr. David Alan Gilbert / [email protected] / Manchester, UK
