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Date: Thu, 18 Jul 2013 11:03:39 -0700
From: Ian Main <imain@redhat.com>
Message-ID: <20130718180339.GA13675@gate.mains.priv>
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Subject: Re: [Qemu-devel] [PATCH V4 4/4] Change default to qcow2 for sync
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To: Eric Blake <eblake@redhat.com>
Cc: qemu-devel@nongnu.org

On Thu, Jul 18, 2013 at 11:32:52AM -0600, Eric Blake wrote:
> On 07/18/2013 11:27 AM, Eric Blake wrote:
> 
> >>      if (!has_format) {
> >> -        format = mode == NEW_IMAGE_MODE_EXISTING ? NULL : bs->drv->format_name;
> >> +        format = mode == NEW_IMAGE_MODE_EXISTING ? NULL : "qcow2";
> > 
> > Is this the right thing to do?  Or should we do:
> > 
> > if (!has_format) {
> >     if (mode == NEW_IMAGE_MODE_EXISTING) {
> >         format = NULL;
> >     } else {
> >         format = bs->drv->format_name ?: "qcow2";
> >     }
> > }
> > 
> > That is, I think we should default to doing a backup in the format given
> > by the original (what if the original is qed, which also supports
> > backing files), and only use qcow2 when there is no guidance whatsoever.
> > 
> > But in practice, I don't care
> 
> Well, I _DO_ care about one thing - make sure that the qapi-schema.json
> page accurately documents how this variable is defaulted for callers
> that don't care about the implications of omitting a format.
> 
> Or we could simplify life by making 'format' mandatory for drive-backup;
> it was optional for 'drive-mirror' due to incremental implementation,
> but for 'drive-backup', we still have the opportunity to do things right
> from the first release.

Ah, I did make a doc change, I must have forgotten to add it.

I'm all for making format mandatory if that is ok with everyone.  Maybe
that is the best solution.

	Ian