> IIUC, he already uses a refcount table. Actually, I think that a > refcount table is a requirement to provide the interesting properties > that internal snapshots have (see my other mail). > > Refcount tables aren't a very complex thing either. In fact, it makes a > format much simpler to have one concept like refcount tables instead of > adding another different mechanism for each new feature that would be > natural with refcount tables. > > The only problem with them is that they are metadata that must be > updated. However, I think we have discussed enough how to avoid the > greatest part of that cost.
FVD's novel uses of the reference count table reduces the metadata update overhead down to literally zero during normal execution of a VM. This gets the bests of QCOW2's reference count table but without its oeverhead. In FVD, the reference count table is only updated when creating a new snapshot or deleting an existing snapshot. The reference count table is never updated during normal execution of a VM.
