Re: [Qemu-block] [PATCH COLO v3 01/14] docs: block replication's description

[Kevin Wolf]

Am 23.04.2015 um 11:26 hat Paolo Bonzini geschrieben:
> 
> 
> On 23/04/2015 11:00, Kevin Wolf wrote:
> > Because it may be the right design.
> > 
> > If you're really worried about the test matrix, put a check in the
> > filter block driver that its bs->file is qcow2. Of course, such an
> > artificial restriction looks a bit ugly, but using a bad design just
> > in order to get the same restriction is even worse.
> > 
> > Stefan originally wanted to put image streaming in the QED driver. I
> > think we'll agree today that it was right to reject that. It's simply
> > not functionality related to the format. Adding replication logic to
> > qcow2 looks similar to me in that respect.
> 
> Yes, I can't deny it is similar.  Still, there is a very important
> difference: limiting colo's internal workings to qcow2 or NBD doesn't
> limit what the user can do (while streaming limited the user to image
> files in QED format).
> 
> It may also depend on how the patches look like and how much the colo
> code relies on other internal state.
> 
> For NBD the answer is almost nothing, and you don't even need a filter
> driver.  You only need to separate sharply the "configure" and "open"
> phases.  So it may indeed be possible to generalize the handling of the
> secondary to non-NBD.
> 
> It may be the same for the primary; I admit I haven't even tried to read
> the qcow2 patch, as I couldn't do a meaningful review.

The qcow2 patch only modifies two existing lines. The rest it adds is a
the qcow2+colo BlockDriver, which references some qcow2 functions
directly and has a wrapper for others. On a quick scan, it didn't seem
like it accesses any internal qcow2 variables or calls any private
functions.

In other words, it's the perfect example for a filter.

Kevin