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

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

[Paul Schlacter]

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 ?

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

[Dr. David Alan Gilbert]

* 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

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

[Paul Schlacter]

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
>>
>
>

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

[Paul Schlacter]

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?

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
>>>
>>
>>
>

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

[Dr. David Alan Gilbert]

* 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

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

[Stefan Hajnoczi]

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On Fri, Nov 03, 2017 at 09:10:45AM +0000, Dr. David Alan Gilbert wrote:
> * 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)

Please provide full details of the problem you are observing:

1. qemu command-line on the host (ps aux | grep qemu)
2. dd command-line inside the guest
3. "Good" output and "bad" output

Stefan

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