Re: [Qemu-devel] qemu disk use writeback don't take effect in centos 7.2 1511

["Dr. David Alan Gilbert" <dgilbert@redhat.com>]

* Paul Schlacter (wlfightup@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 <wlfightup@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 <wlfightup@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 <
> >> dgilbert@redhat.com> wrote:
> >>
> >>> * Paul Schlacter (wlfightup@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 / dgilbert@redhat.com / Manchester, UK
> >>>
> >>
> >>
> >
--
Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK