From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from [140.186.70.92] (port=33165 helo=eggs.gnu.org) by lists.gnu.org with esmtp (Exim 4.43) id 1P9Hlm-0006Uw-IK for Qemu-devel@nongnu.org; Fri, 22 Oct 2010 09:34:55 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1P9Hlk-0000gT-W6 for Qemu-devel@nongnu.org; Fri, 22 Oct 2010 09:34:54 -0400 Received: from mx1.redhat.com ([209.132.183.28]:1025) by eggs.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1P9Hlk-0000g9-ND for Qemu-devel@nongnu.org; Fri, 22 Oct 2010 09:34:52 -0400 Message-ID: <4CC19322.7050205@redhat.com> Date: Fri, 22 Oct 2010 15:35:30 +0200 From: Kevin Wolf MIME-Version: 1.0 Subject: Re: [Qemu-devel] bdrv_flush for qemu block drivers nbd, rbd and sheepdog References: <4CC04920.8040902@redhat.com> <4CC05744.1060204@codemonkey.ws> <1287689572.2724.0.camel@Quad> <4CC14B60.7090900@redhat.com> <4CC18A89.1060802@codemonkey.ws> In-Reply-To: <4CC18A89.1060802@codemonkey.ws> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit List-Id: qemu-devel.nongnu.org List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: Anthony Liguori Cc: Laurent Vivier , Qemu-devel@nongnu.org Am 22.10.2010 14:58, schrieb Anthony Liguori: > On 10/22/2010 03:29 AM, Kevin Wolf wrote: >>> I agree. >>> >> Of course, as Laurent said a while ago, there is no specification for >> NBD, so it's hard to say what the intended semantics is. >> >> However, I did have a look at the nbdserver code and it looks as if it >> implements something similar to writethrough (namely fsync after each >> write) only if configured this way on the server side. qemu-nbd defaults >> to writethrough, but can be configured to use cache=none. So with either >> server qemu as a client can't tell whether the data is safe on disk or not. >> >> In my book this is a strong argument for refusing to open nbd >> connections with anything but cache=unsafe. >> > > On a physical system, if you don't have a battery backed disk and you > enable the WC on your disk, then even with cache=writethrough we're unsafe. I don't think that's right. O_SYNC should guarantee that the volatile disk cache is flushed. > Likewise, if you mount your filesystem with barrier=0, QEMU is unsafe. Yeah, if you do something equivalent to cache=unsafe on a lower layer, then qemu can't do much about it. Maybe you can apply the same argument to NBD, even though it's unsafe by default. > QEMU can't guarantee safety. The underlying storage needs to be > configured correctly. As long as we're not introducing caching within > QEMU, I don't think we should assume we're unsafe. > > Do we have any place where we can add docs on a per-block format basis? > It would be good to at least mention for each block device how the > backing storage needed to be configured for safety. docs/block-protocols.txt? Kevin