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Re: [Qemu-block] [Qemu-devel] [PATCH] qcow2: Optimize the refcount-block
From: |
Eric Blake |
Subject: |
Re: [Qemu-block] [Qemu-devel] [PATCH] qcow2: Optimize the refcount-block overlap check |
Date: |
Mon, 30 Jan 2017 12:27:44 -0600 |
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Mozilla/5.0 (X11; Linux x86_64; rv:45.0) Gecko/20100101 Thunderbird/45.6.0 |
On 01/30/2017 10:34 AM, Alberto Garcia wrote:
>
> I don't think QEMU produces files where refcount_table[i] == 0 but
> refcount_table[i + 1] != 0. Do they even make sense?
I don't know if qemu can be directly coerced to create such an image,
but a third party tool can (and that probably includes qemu-io), and
such an image makes sense. In particular, a refcount can be changed
from non-zero to zero during garbage collection when all clusters
covered by that refcount are no longer in use (perhaps because a
snapshot was deleted).
> In any case, my
> patch would cover those cases too, but this simplified version wouldn't.
Since an image can have holes in the refcount_table, it sounds like the
simplified version is not robust enough.
--
Eric Blake eblake redhat com +1-919-301-3266
Libvirt virtualization library http://libvirt.org
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