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
signature.asc
Description: OpenPGP digital signature
