On Mon 06 Aug 2018 01:30:33 PM CEST, Kevin Wolf wrote: > Am 06.08.2018 um 13:07 hat Alberto Garcia geschrieben: >> On Mon 06 Aug 2018 12:45:20 PM CEST, Kevin Wolf wrote: >> > Am 06.08.2018 um 09:47 hat Alberto Garcia geschrieben: >> >> On Fri 03 Aug 2018 04:55:42 PM CEST, Kevin Wolf wrote: >> >> > By the way, weren't you working on subclusters a while ago? How did >> >> > that go? Because I think those would enable us to use larger >> >> > cluster sizes and therefore reduce the metadata sizes as well. >> >> >> >> I had a working prototype, but the changes to both the code and the >> >> on-disk format were not trivial. I would need to re-evaluate its >> >> performance impact after all the changes that we have had since then, >> >> and then see if it's worth trying again. >> >> >> >> I suppose that the main benefit of having subclusters is that >> >> allocations are much faster. Everything else remains more or less the >> >> same, and in particular you can already use larger clusters if you >> >> want to reduce the metadata sizes. Plus, with the l2-cache-entry-size >> >> option you can already solve some of the problems of having large >> >> clusters. >> > >> > Yes, indeed, subclusters are about making COW less painful (or getting >> > rid of it altogether). Doing COW for full 2 MB when the guest updates >> > 4k is just a bit over the top and I think it slows down initial writes >> > seriously. I haven't benchmarked things in a while, though. >> >> Me neither, I think the most recent ones that I have are from last year: >> >> https://lists.gnu.org/archive/html/qemu-devel/2017-04/msg01033.html >> >> Since then we changed the cow algorithm to do only 2 operations instead >> of 5, so that can affect the best case scenario. > > I can't find the numbers from the COW request merging any more, but I > remember that when I tested the approach, the result was something > like this: Merging the write requests pretty much got rid of any > measurable COW overhead without a backing file, but as soon as you > have to read from the backing file, the saved work isn't very > significant any more and the cost of the read dominates. > > So when you test subclusters, be sure to not only test an empty > standalone qcow2 image, but also a newly taken snapshot that has to > COW from the backing file a lot.
Yes, that's the use case that I had in mind too. Berto
