On Fri 28 Jun 2019 04:16:28 PM 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. >> >> I wonder if it would be possible to have a hybrid solution: >> >> With 64 bits to indicate the allocation status of each subcluster we >> still have the original L2 entry with its QCOW_OFLAG_ZERO bit, so we >> need to specify what happens if that bit is set and at the same time >> some subclusters are marked as allocated. >> >> One possibility is that allocated subclusters have actual data and >> the rest are zero subclusters. > > Hm, yes, that would be possible. > > However, that would require some addtional complexity in write_zeroes > then: If the zero flag isn't set yet, then we need to check that no > other subcluster is unallocated before we can turn the zero flag on > for the whole cluster. You couldn't have subclusters referring to the > backing file and zeroed subclusters at the same time. > > Maybe doubling the number of bits would actually be worth the > additional complexity in write_zeroes.
Yes, that's a bit my doubt. How important it is to have those features that you mention vs being able to halve the minimum unit of allocation. Berto
