From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from [140.186.70.92] (port=35389 helo=eggs.gnu.org) by lists.gnu.org with esmtp (Exim 4.43) id 1P9I7M-0002ne-75 for Qemu-devel@nongnu.org; Fri, 22 Oct 2010 09:57:14 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1P9I7L-0006ep-5H for Qemu-devel@nongnu.org; Fri, 22 Oct 2010 09:57:12 -0400 Received: from mx1.redhat.com ([209.132.183.28]:45153) by eggs.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1P9I7K-0006eg-RF for Qemu-devel@nongnu.org; Fri, 22 Oct 2010 09:57:11 -0400 Message-ID: <4CC19855.6050801@redhat.com> Date: Fri, 22 Oct 2010 15:57:41 +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> <4CC19322.7050205@redhat.com> <4CC1956C.9080008@codemonkey.ws> In-Reply-To: <4CC1956C.9080008@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 15:45, schrieb Anthony Liguori: >>> 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. >> > > If your filesystem does the right thing which an awful lot of them don't > today. The list of really relevant filesystems is rather short, though. >>> 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? >> > > Maybe docs/block/.txt? Would be a good home for the qed spec too. I think spec and documentation for users should be kept separate. I thought that's the reason why docs/specs/ exists. And if you exclude specs, I'm not sure if there's a lot left to say for each format. Having ten files under docs/block/ which consist of two lines each would be ridiculous. If contrary to my expectations we actually do have content for it, docs/block/.txt works for me as well. Kevin