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


From: Stefan Hajnoczi
Subject: Re: [Qemu-devel] qemu disk use writeback don't take effect in centos 7.2 1511
Date: Fri, 3 Nov 2017 10:09:12 +0000
User-agent: Mutt/1.9.1 (2017-09-22)

On Fri, Nov 03, 2017 at 09:10:45AM +0000, Dr. David Alan Gilbert wrote:
> * Paul Schlacter (address@hidden) wrote:
> > latest progress´╝Ü
> > 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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