Re: [Qemu-block] [Qemu-devel] [PATCH for-2.12 0/4] qmp dirty bitmap API

[Kashyap Chamarthy]

On Thu, Dec 07, 2017 at 06:33:04PM +0100, Kevin Wolf wrote:
> Am 07.12.2017 um 12:56 hat Kashyap Chamarthy geschrieben:

[...]

> > So from the above interesting discussion, it seems like Kevin is leaning
> > towards a single job type that offers 'stream', 'commit', 'backup', and
> > 'mirror' functionality as part of a single command / job type.  Based on
> > an instinct, this sounds a bit too stuffy and complex to me.
> > 
> > And John seems to be leaning towards two block device job types:
> > 
> >   - 'blockdev-foo' that offers both current 'stream' and 'commit'
> >     functionality as two different options to the same QMP command; and
> > 
> >   - 'blockdev-bar' will offer both 'mirror' and 'backup' functionality
> >     as part of the same QMP command
> > 
> > FWIW, this seems a bit more palatable, as it is unifying
> > similar-functionality-that-differ-slightly into two distinct commands.

[...]

> Commit of the active layer is _already_ a mirror job internally (and not
> a stream job). 

I see; didn't realize the above.  I learn that it is the case so for a
long time, and came in via the commit 20a63d2 ("commit: Support commit
active layer").  For my own education, I went looking into
qemu/block/mirror.c, and noticed the variable 'commit_active_job_driver'
(of type BlockJobDriver), which is used in the function:
commit_active_start(). 

Thanks for the pointer.

> It's pretty clear to me that commit and mirror are almost
> the same, backup is pretty similar. Stream is somewhat different and
> might make more sense as a separate job type.

Yep, understood.

-- 
/kashyap