From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([208.118.235.92]:55483) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1TtAqR-00008R-Ht for qemu-devel@nongnu.org; Thu, 10 Jan 2013 00:38:28 -0500 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1TtAqO-0003Hu-HM for qemu-devel@nongnu.org; Thu, 10 Jan 2013 00:38:25 -0500 Received: from mail-pb0-f52.google.com ([209.85.160.52]:51678) by eggs.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1TtAqO-0003Hm-Aa for qemu-devel@nongnu.org; Thu, 10 Jan 2013 00:38:24 -0500 Received: by mail-pb0-f52.google.com with SMTP id ro2so100335pbb.39 for ; Wed, 09 Jan 2013 21:38:23 -0800 (PST) Message-ID: <50EE53C8.4090009@gmail.com> Date: Thu, 10 Jan 2013 13:38:16 +0800 From: Liu Yuan 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> <50ED45A8.5020706@redhat.com> <50ED4829.1070302@gmail.com> <50ED4933.3040001@redhat.com> <50ED4A90.2080808@gmail.com> <50ED5D64.4040600@gmail.com> <50ED65AA.60000@redhat.com> <50ED6AF4.4060300@gmail.com> <50ED8868.6010805@redhat.com> In-Reply-To: <50ED8868.6010805@redhat.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: Paolo Bonzini Cc: Kevin Wolf , Stefan Hajnoczi , qemu-devel@nongnu.org, MORITA Kazutaka On 01/09/2013 11:10 PM, Paolo Bonzini wrote: > Il 09/01/2013 14:04, Liu Yuan ha scritto: >>>> 2 The upper layer software which relies on the 'cache=xxx' to choose >>>> cache mode will fail its assumption against new QEMU. >>> >>> Which assumptions do you mean? As far as I can say the behaviour hasn't >>> changed, except possibly for the performance. >> >> When users set 'cache=writethrough' to export only a writethrough cache >> to Guest, but with new QEMU, it will actually get a writeback cache as >> default. > > They get a writeback cache implementation-wise, but they get a > writethrough cache safety-wise. How the cache is implemented doesn't > matter, as long as it "looks like" a writethrough cache. > > In fact, consider a local disk that doesn't support FUA. In old QEMU, > images used to be opened with O_DSYNC and that splits each write into > WRITE+FLUSH, just like new QEMU. All that changes is _where_ the > flushes are created. Old QEMU changes it in the kernel, new QEMU > changes it in userspace. > >> We don't need to communicate to the guest. I think 'cache=xxx' means >> what kind of cache the users *expect* to export to Guest OS. So if >> cache=writethrough set, Guest OS couldn't turn it to writeback cache >> magically. This is like I bought a disk with 'writethrough' cache >> built-in, I didn't expect that it turned to be a disk with writeback >> cache under the hood which could possible lose data when power outage >> happened. > > It's not by magic. It's by explicitly requesting the disk to do this. > > Perhaps it's a bug that the cache mode is not reset when the machine is > reset. I haven't checked that, but it would be a valid complaint. > Ah I didn't get the current implementation right. I tried the 3.7 kernel and it works as expected (cache=writethrough result in a 'writethrough' cache in the guest). It looks fine to me to emulate writethrough as writeback + flush, since the profermance drop isn't big, though sheepdog itself support true writethrough cache (no flush). I am going to send v2 of directio patch for sheepdog driver. Thanks, Yuan