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

[Liang Li]

On Thu, Jan 25, 2018 at 08:48:22AM -0600, Eric Blake wrote:
> On 01/24/2018 10:59 PM, Liang Li wrote:
> >>
> >> 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
> >>
> > Hi Eric,
> > 
> > Thinks for your information, I didn't know libvirt depends on 
> > 'block-job-cancel'
> > for some of the block related operations.
> > 
> > It's seems a new interface should provided by qemu for use case that just
> > for aborting block job and don't care abort the mirror data integrality, and
> > libvirt can make use of this new interface.
> > 
> > Do you think this is the right direction?
> 
> I don't know if it is better to wait for the new async mirroring code to
> land, or to just propose a new QMP command that can force-quit an
> ongoing mirror in the READY state, but you are correct that the only
> safe way to do it is by adding a new command (or a new optional flag to
> the existing block-job-cancel command).
> 

Active sync does not conflict with the new QMP command, no need to wait.
The current QMP command is:

{ 'command': 'block-job-cancel', 'data': { 'device': 'str', '*force': 'bool' } }

'force' has other meaning which is not used by libvirt, for the change, there
are 3 options:

a. Now that 'force' is not used by libvirt and it current semantic is not very 
useful,
we can change it's semantic to force-quit without syncing.

b. change 'force' from bool to flag, and bit 0 is used for it's original 
meaning.

c. add another bool parameter.


which is the best one?

Thanks!

Liang 


> -- 
> Eric Blake, Principal Software Engineer
> Red Hat, Inc.           +1-919-301-3266
> Virtualization:  qemu.org | libvirt.org
>