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


From: Paolo Bonzini
Subject: Re: [Qemu-devel] [PATCH COLO v3 01/14] docs: block replication's description
Date: Thu, 23 Apr 2015 11:26:23 +0200
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:31.0) Gecko/20100101 Thunderbird/31.5.0


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.

Paolo



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