Re: [Qemu-block] [Qemu-devel] [PATCH] block/mirror: fix fail to cancel when VM has heavy BLK IO

[John Snow]

On 01/24/2018 02:16 PM, Eric Blake wrote:
> On 01/24/2018 12:17 AM, Liang Li wrote:
>> We found that when doing drive mirror to a low speed shared storage,
>> if there was heavy BLK IO write workload in VM after the 'ready' event,
>> drive mirror block job can't be canceled immediately, it would keep
>> running until the heavy BLK IO workload stopped in the VM. This patch
>> fixed this issue.
> 
> I think you are breaking semantics here.  Libvirt relies on
> 'block-job-cancel' after the 'ready' event to be a clean point-in-time
> snapshot, but that is only possible if there is no out-of-order pending
> I/O at the time the action takes place.  Breaking in the middle of the
> loop, without using bdrv_drain(), risks leaving an inconsistent copy of
> data in the mirror not corresponding to any point-in-time on the source.
> 
> There's ongoing work on adding async mirroring; this may be a better
> solution to the issue you are seeing.
> 
> https://lists.gnu.org/archive/html/qemu-devel/2018-01/msg05419.html
> 

Sounds like another point for the idea of using a "completion mode" in a
2.0 API instead of treating "cancel" like a valid way of completing a job.

(Kevin: If you're taking this up, it would be *very* nice to have jobs
have an option via job-set-property or some such command that allows us
to change our desired completion mode on the fly, which frees up cancel
to be simply a cancel.)

--js