Re: [Qemu-devel] QCOW2 deduplication design

[Eric Blake <eblake@redhat.com>]

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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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