Re: [Qemu-devel] QCOW2 deduplication design

[Stefan Hajnoczi]

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