From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([208.118.235.92]:56805) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1TssrG-0006it-Em for qemu-devel@nongnu.org; Wed, 09 Jan 2013 05:26:07 -0500 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1TssrE-0004Po-Tl for qemu-devel@nongnu.org; Wed, 09 Jan 2013 05:26:05 -0500 Received: from mx1.redhat.com ([209.132.183.28]:42263) by eggs.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1TssrE-0004Pk-Jg for qemu-devel@nongnu.org; Wed, 09 Jan 2013 05:26:04 -0500 Message-ID: <50ED45A8.5020706@redhat.com> Date: Wed, 09 Jan 2013 11:25:44 +0100 From: Paolo Bonzini MIME-Version: 1.0 References: <1355941771-3418-1-git-send-email-namei.unix@gmail.com> <87k3s6shdv.wl%morita.kazutaka@lab.ntt.co.jp> <50D967C3.7020109@gmail.com> <50E58B19.2050701@gmail.com> <20130104163830.GF6310@stefanha-thinkpad.hitronhub.home> <50E7AEC4.5080309@gmail.com> <50E7BA41.3020307@gmail.com> <50E7DC9B.4080309@gmail.com> <50EACC61.2020603@redhat.com> <50EBB1CB.9030608@gmail.com> <20130108094025.GE2557@stefanha-thinkpad.redhat.com> <50EBEAD2.6070608@gmail.com> <50EBEE42.7010407@redhat.com> <50EBF755.3050607@gmail.com> <50EBFA3F.8030808@redhat.com> <50EBFE20.9010100@gmail.com> <50EC00CE.80205@redhat.com> <50EC0493.8030701@gmail.com> <50EC0D41.4070200@redhat.com> <50EC1C9A.5040006@gmail.com> In-Reply-To: <50EC1C9A.5040006@gmail.com> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit Subject: Re: [Qemu-devel] [PATCH] sheepdog: implement direct write semantics List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: Liu Yuan Cc: Kevin Wolf , Stefan Hajnoczi , qemu-devel@nongnu.org, MORITA Kazutaka Il 08/01/2013 14:18, Liu Yuan ha scritto: > Maybe not for a second thought. See following combination: > > cache flags WCE toggled and resulting behavior > writethrough writethrough > writeback writetrhough (writeback + flush as expected) > > cache flags means specify 'cache=xxx' at startup and WCE toggled on the > fly in the guest (supose guest kernel support WCE control) > > So the result is *not* broken. If we set cache=writethrough for > sheepdog, then WCE won't take any effect because 'flush' request will be > ignored by Sheepdog driver. And with cache=writeback, WCE does disable > the writecache and actually turns it to a writethrough cache by sending > flush req every time for write. > > To conclude, let Sheepdog interpret cache flags won't cause trouble even > with current Guest WCE feature, the different is that if we set > cache=writethrough, guest can't change it via WCE toggling. Is this > behavior acceptable? But why is it useful to force-disable writeback caching? Do you have any performance numbers? Paolo