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Re: [Qemu-devel] QCOW2 deduplication design
From: |
Eric Blake |
Subject: |
Re: [Qemu-devel] QCOW2 deduplication design |
Date: |
Wed, 09 Jan 2013 09:32:18 -0700 |
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Mozilla/5.0 (X11; Linux x86_64; rv:17.0) Gecko/17.0 Thunderbird/17.0 |
On 01/09/2013 09:16 AM, Stefan Hajnoczi wrote:
>> I.6) max refcount reached
>> The L2 hash block of the cluster is written in order to remember at next
>> startup
>> that it must not be used anymore for deduplication. The hash is dropped from
>> the
>> gtrees.
>
> Interesting case. This means you can no longer take snapshots
> containing this cluster because we cannot track references :(.
>
> Worst case: guest fills the disk with the same 4 KB data (e.g.
> zeroes). There is only a single data cluster but the refcount is
> maxed out. Now it is not possible to take a snapshot.
Except that a sector of all zeroes should be represented as a hole,
rather than occupying a cluster (that is, since all zeros is the most
likely value for a very common data, but also has a special case
representation that doesn't take any clusters at all, we should take
advantage of that). But your point remains for a disk that reuses
another common 4KB cluster more than the refcount allows (for example,
how many copies of COPYING do you have on your disk?).
--
Eric Blake eblake redhat com +1-919-301-3266
Libvirt virtualization library http://libvirt.org
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