Re: [Qemu-devel] [PATCH v2 resend] block/mirror: change the semantic of 'force' of block-job-cancel

[Jeff Cody <jcody@redhat.com>]

On Fri, Mar 02, 2018 at 11:20:55AM -0500, John Snow wrote:
> 
> 
> On 03/02/2018 10:39 AM, Eric Blake wrote:
> > On 02/26/2018 08:05 PM, Liang Li wrote:
> >> 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.
> >>
> >> Libvirt depends on the current block-job-cancel semantics, which is that
> >> when used without a flag after the 'ready' event, the command blocks
> >> until data is in sync.  However, these semantics are awkward in other
> >> situations, for example, people may use drive mirror for realtime
> >> backups while still wanting to use block live migration.  Libvirt cannot
> >> start a block live migration while another drive mirror is in progress,
> >> but the user would rather abandon the backup attempt as broken and
> >> proceed with the live migration than be stuck waiting for the current
> >> drive mirror backup to finish.
> >>
> >> The drive-mirror command already includes a 'force' flag, which libvirt
> >> does not use, although it documented the flag as only being useful to
> >> quit a job which is paused.  However, since quitting a paused job has
> >> the same effect as abandoning a backup in a non-paused job (namely, the
> >> destination file is not in sync, and the command completes immediately),
> >> we can just improve the documentation to make the force flag obviously
> >> useful.
> > 
> > How does this interact with John's ongoing work to redo block job
> > semantics:
> > https://lists.gnu.org/archive/html/qemu-devel/2018-02/msg06167.html
> > 
> >>
> >> Cc: Paolo Bonzini <pbonzini@redhat.com>
> >> Cc: Jeff Cody <jcody@redhat.com>
> >> Cc: Kevin Wolf <kwolf@redhat.com>
> >> Cc: Max Reitz <mreitz@redhat.com>
> >> Cc: Eric Blake <eblake@redhat.com>
> >> Cc: John Snow <jsnow@redhat.com>
> >> Reported-by: Huaitong Han <huanhuaitong@didichuxing.com>
> >> Signed-off-by: Huaitong Han <huanhuaitong@didichuxing.com>
> >> Signed-off-by: Liang Li <liliangleo@didichuxing.com>
> >> ---
> > 
> > But in isolation, the patch looks reasonable to me.
> > 
> > Reviewed-by: Eric Blake <eblake@redhat.com>
> > 
> 
> In fairness, this patch hit the list before mine did, so...
> 
> I think it'll be OK to accommodate -- I think. It just changes the
> nature of how fast the cancellation happens in mirror, which shouldn't
> muck around with the general flow of job management in general. I think.
> 
> Famous last words.
>

I think Kevin's pulled your series in through his branch, and this patch
conflicts with it.  Do we want to try to rebase this patch on top of your
series?