Am 28.06.2019 um 11:53 hat Alberto Garcia geschrieben: > On Fri 28 Jun 2019 11:20:57 AM CEST, Kevin Wolf wrote: > >> >> I would consider 64k cluster/8k subcluster as too extreme for me. > >> > >> I forgot to add: this 64k/8k ratio is only with my current prototype. > >> > >> In practice if we go with the 128-bit L2 entries we would have 64 > >> subclusters per cluster, or 32 if we want to have a separate > >> QCOW_OFLAG_ZERO for each subcluster (would we need this?). > > > > Yes, I think we'd want to have a separate zero flag for each > > subcluster. Otherwise, when writing to a zero cluster, you'd have to > > COW the whole supercluster again. > > Yes if the original cluster had the QCOW_OFLAG_ZERO bit, not if it was > simply unallocated.
Right, but that leaving clusters simply unallocated doesn't quite cut it is something we already noticed before writing the spec for v3. Only really necessary when you have a backing file, of course, but that's one of the more interesting cases for subclusters anyway. Kevin
