* Paul Schlacter (wlfigh...@gmail.com) wrote: > latest progress： > use centos 7.2 https://cloud.centos.org/centos/7/images/ > test version: 1511 1601 1604 1606 1608 > > From the above version in turn test, found the previous version 1511 1601 > 1603 have the problem, > The latter two versions 1606 1608 are normal, dont't have the prblem, > Normal is cache = writeback io significantly higher than none mode > > why?
It's a fun bug, but I don't know the cause - the fact it's dependent on guest version but fixed, suggests something changed in the guest kernel. If you really wanted to figure out why I guess the only way would be to walk through the CentOS guest kernel versions and find the exact kernel that fixed it; then we could hunt down which change fixed it. (I've cc'd in Stefan for block stuff, but this is sounding more guest than qemu) Dave > On Wed, Nov 1, 2017 at 8:03 PM, Paul Schlacter <wlfigh...@gmail.com> wrote: > > > Hi David: > > yes, with the same qemu, centos 7.2 guest cache is writeback and > > none, the disk io almost, writeback feeling no effect. > > but 7.1/7.3/7.4 cache is writeback was significantly faster than > > none model > > > > On Wed, Nov 1, 2017 at 8:02 PM, Paul Schlacter <wlfigh...@gmail.com> > > wrote: > > > >> Hi David: > >> yes, with the same qemu, centos 7.2 guest cache is writeback and > >> none, the disk io almost, writeback feeling no effect. > >> but 7.1/7.3/7.4 cache is writeback was significantly faster than > >> none model > >> > >> On Wed, Nov 1, 2017 at 5:45 PM, Dr. David Alan Gilbert < > >> dgilb...@redhat.com> wrote: > >> > >>> * Paul Schlacter (wlfigh...@gmail.com) wrote: > >>> > qemu version: 2.9.0 > >>> > image: centos 7.2 1511 raw > >>> > cache mode: writeback > >>> > > >>> > > >>> > In centos7.2 time, the use of writeback and none, the result of dd > >>> disk is > >>> > the same io, no change > >>> > > >>> > But I replaced centos7.1 centos7.3 centos7.4 when writeback was > >>> > significantly faster than none > >>> > > >>> > Would like to ask what is going on, how to solve ? > >>> > >>> Hi Paul, > >>> Can you just clarify your setup please; do you mean that > >>> with the same qemu setup but a 7.2 guest rather than a 7.1/7.3 guest > >>> the behaviour changes? > >>> > >>> Dave > >>> > >>> -- > >>> Dr. David Alan Gilbert / dgilb...@redhat.com / Manchester, UK > >>> > >> > >> > > -- Dr. David Alan Gilbert / dgilb...@redhat.com / Manchester, UK
