From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from [140.186.70.92] (port=43861 helo=eggs.gnu.org) by lists.gnu.org with esmtp (Exim 4.43) id 1P9Hvi-0003Gb-Iw for Qemu-devel@nongnu.org; Fri, 22 Oct 2010 09:45:12 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1P9Hvg-0003Y0-Ow for Qemu-devel@nongnu.org; Fri, 22 Oct 2010 09:45:10 -0400 Received: from mail-gy0-f173.google.com ([209.85.160.173]:44595) by eggs.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1P9Hvg-0003Xs-Jm for Qemu-devel@nongnu.org; Fri, 22 Oct 2010 09:45:08 -0400 Received: by gya6 with SMTP id 6so667650gya.4 for ; Fri, 22 Oct 2010 06:45:07 -0700 (PDT) Message-ID: <4CC1956C.9080008@codemonkey.ws> Date: Fri, 22 Oct 2010 08:45:16 -0500 From: Anthony Liguori 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> In-Reply-To: <4CC19322.7050205@redhat.com> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit List-Id: qemu-devel.nongnu.org List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: Kevin Wolf Cc: Laurent Vivier , Qemu-devel@nongnu.org On 10/22/2010 08:35 AM, Kevin Wolf wrote: > 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. > If your filesystem does the right thing which an awful lot of them don't today. >> 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. Regards, Anthony Liguori > Kevin >