Re: [libvirt] Querying block device job status and semantics of virDomainBlockJobInfo()

On 09/01/2016 08:57 AM, Kashyap Chamarthy wrote: > So, I'm trying to understand how libvirt reports the "cur" and "end" > values. I've read the virDomainBlockJobInfo() struct, it wasn't crystal > clear. It states: > > /* > * The following fields provide an indication of block job progress. @cur > * indicates the current position and will be between 0 and @end. @end is > * the final cursor position for this operation and represents completion. > * To approximate progress, divide @cur by @end. > */ Libvirt is (more or less) reporting numbers from qemu. What's more, @end need not be the same between calls; qemu is free to change the end value as it comes up with more work to do (or sees that less work remains than initially estimated), all that REALLY matters is that the ratio between the two numbers converges, and is < 1 while busy, and == 1 when complete. Except that there are cases in qemu where a block job really has 0 work to do. Prior to qemu exposing the "ready":true/false flag, libvirt had to guess whether equal numbers really meant done, or if it merely meant nearly done. But now that we have "ready", I think the sanest course of action for libvirt is to fudge the numbers from qemu. After all, we've already documented (both in libvirt and in qemu) that @end is not fixed, so much as a moving target (it just doesn't move very much on operations where we have a good grasp on how much work remains from the start, like a deep copy; but is more prone to move on operations like live commit that are influenced by how active the guest is at writing data that we are attempting to commit at the same time).

> [kashyap] > > So, if the job hasn't started yet, what should libvirt report? > > [mprivozn] > > That's the question. We can't change the [virDomainBlockJobInfo] > struct (otherwise we won't be ABI compatible), so we can't really > add a bolean there 'bool job_started'. > > Or we can introduce new "job type" which wouldn't really be a job > type, but we will fill status.job with it to say explicitly job > hasn't started yet > > > So, any other thoughts here, on how to proceed here? My preference would be: If qemu doesn't report anything (because the job is not started yet), then libvirt should report cur=0, end=1 (the job still has 100% to go). If qemu reports 0/0 and "done":false, then libvirt should report cur=0, end=1 (that is, we fudge the end to be larger, because the job is not done yet). If qemu reports 0/0 and "done":true (because the job was really a no-op), then libvirt should report cur=1, end=1 (the job is 100% complete). If qemu reports 0/0 and lacks "done" (older qemu), then libvirt just has to guess. I'm not sure which guess is most appropriate; maybe libvirt itself will have to set up a timer and report 0/1 the first time, and only report 1/1 after a minimum time has elapsed, to make sure qemu has had a chance to do something about the job. Or maybe we don't worry about it, and just have libvirt report 0/0 because we really don't know any better. If qemu reports a/b, where a < b and b > 0, use those numbers as is. We don't even have to check "done". If qemu reports a/a, where a > 0, then also check "done". If "done":false is present, report a-1/a (the job is not quite done); if "done" is absent or "done":true is present, report a/a (the job is done).