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Re: [Qemu-block] [PATCH for-2.12 v2] qcow2: Reset free_cluster_index whe

From: Kevin Wolf
Subject: Re: [Qemu-block] [PATCH for-2.12 v2] qcow2: Reset free_cluster_index when allocating a new refcount block
Date: Wed, 21 Mar 2018 16:40:00 +0100
User-agent: Mutt/1.9.1 (2017-09-22)

Am 21.03.2018 um 16:32 hat Alberto Garcia geschrieben:
> On Wed 21 Mar 2018 04:07:28 PM CET, Kevin Wolf wrote:
> > I just remembered that when I looked at an image recently, I noticed
> > that the refcount block wasn't in the first cluster and I disliked it,
> > though mostly because it felt untidy rather than being a problem.
> I don't think you can fix that in general. In my test case I'm writing
> new data when the existing refcount block is already full, so we can
> allocate the new one before the new data and keep everything tidy.
> But if there are, say, two refcount entries available and you write four
> data clusters you don't want the new refcount block in the middle of
> those four clusters just to have it at the beginning :-)

Agreed. I think if implemented like I suggested (hard error in
update_refcount() and then allocate a new refcount block in the caller),
we'd actually end up with the refcount block covered by an old refcount
block rather than being self-describing. And that in turn wouldn't make
things much tidier, so I guess you're right. :-)

> > Having a qcow2 analysis script in the repo sounds like a good
> > idea. John has something, too. Maybe we can check whether the two
> > things complement each other and then check in a script that combines
> > both (or if one provides a superset of the other, just check in that
> > one).
> I'll take a look.



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