> Am 08.09.2014 um 17:27 schrieb Paolo Bonzini <pbonz...@redhat.com>: > > Il 08/09/2014 17:18, Peter Lieven ha scritto: >>> >>> That's why we have splitting code for discard, and why we would have to >>> add it for read/write too. >> >> Why should a guest generate such big requests. > > When copying data, gparted will try using very large I/O sizes. Of > course if something breaks it will just use a smaller size, but it would > make performance worse. > > I tried now (with local storage, not virtual---but with such large block > sizes it's disk bound anyway, one request can take 0.1 seconds to > execute) and a 2 MB block size is 20% slower than 16 MB block size on > your usual 3.5" rotational SATA disk. >
can you share with what command exactly you ran these tests? i tried myself and found that without multiwrite_merge i was not able to create a request bigger than 0xffff sectors from inside linux. Peter > Paolo > >> Afaik the reported >> limit for e.g. virtio-blk is 1024 sectors (reported through blockdev >> --getmaxsect /dev/vda). >> I think it was only the multiwrite_merge code causing trouble here. > >
