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Re: [Qemu-devel] QCOW2 deduplication design
From: |
Stefan Hajnoczi |
Subject: |
Re: [Qemu-devel] QCOW2 deduplication design |
Date: |
Thu, 10 Jan 2013 07:59:13 +0100 |
On Wed, Jan 9, 2013 at 5:32 PM, Eric Blake <address@hidden> wrote:
> 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,
QEMU doesn't scan data buffers for all zero by default.
Stefan