From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([2001:4830:134:3::10]:55179) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1eAY0B-0003ag-27 for qemu-devel@nongnu.org; Fri, 03 Nov 2017 05:11:00 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1eAY05-0004j8-5a for qemu-devel@nongnu.org; Fri, 03 Nov 2017 05:10:59 -0400 Received: from mx1.redhat.com ([209.132.183.28]:34044) by eggs.gnu.org with esmtps (TLS1.0:DHE_RSA_AES_256_CBC_SHA1:32) (Exim 4.71) (envelope-from ) id 1eAY04-0004iw-TM for qemu-devel@nongnu.org; Fri, 03 Nov 2017 05:10:53 -0400 Date: Fri, 3 Nov 2017 09:10:45 +0000 From: "Dr. David Alan Gilbert" Message-ID: <20171103091045.GA2616@work-vm> References: <20171101094537.GA2786@work-vm> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline In-Reply-To: Content-Transfer-Encoding: quoted-printable Subject: Re: [Qemu-devel] qemu disk use writeback don't take effect in centos 7.2 1511 List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: Paul Schlacter , stefanha@redhat.com Cc: qemu-devel@nongnu.org * Paul Schlacter (wlfightup@gmail.com) wrote: > latest progress=EF=BC=9A > use centos 7.2 https://cloud.centos.org/centos/7/images/ > test version: 1511 1601 1604 1606 1608 >=20 > From the above version in turn test, found the previous version 1511 16= 01 > 1603 have the problem, > The latter two versions 1606 1608 are normal, dont't have the prblem, > Normal is cache =3D writeback io significantly higher than none mode >=20 > 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 wr= ote: >=20 > > Hi David: > > yes, with the same qemu, centos 7.2 guest cache is writeback an= d > > none, the disk io almost, writeback feeling no effect. > > but 7.1/7.3/7.4 cache is writeback was significantly faster th= an > > none model > > > > On Wed, Nov 1, 2017 at 8:02 PM, Paul Schlacter > > wrote: > > > >> Hi David: > >> yes, with the same qemu, centos 7.2 guest cache is writeback a= nd > >> none, the disk io almost, writeback feeling no effect. > >> but 7.1/7.3/7.4 cache is writeback was significantly faster t= han > >> 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 d= d > >>> 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 gues= t > >>> the behaviour changes? > >>> > >>> Dave > >>> > >>> -- > >>> Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK > >>> > >> > >> > > -- Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK