From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([2001:4830:134:3::10]:53754) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1WBljU-0000Ul-3q for qemu-devel@nongnu.org; Fri, 07 Feb 2014 08:44:47 -0500 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1WBljL-0001Ky-J7 for qemu-devel@nongnu.org; Fri, 07 Feb 2014 08:44:40 -0500 Received: from mx.beyond.pl ([92.43.117.49]:37392) by eggs.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1WBljL-0001Kt-Ck for qemu-devel@nongnu.org; Fri, 07 Feb 2014 08:44:31 -0500 Message-ID: <52F4E33B.4070305@beyond.pl> Date: Fri, 07 Feb 2014 14:44:27 +0100 From: =?UTF-8?B?TWFyY2luIEdpYnXFgmE=?= MIME-Version: 1.0 References: <52F46298.8020602@ozlabs.ru> <52F48F3A.40606@redhat.com> <52F4CD3E.1000809@ozlabs.ru> <52F4D5C6.1050206@redhat.com> <52F4D778.6070001@beyond.pl> <52F4E15E.3020109@redhat.com> In-Reply-To: <52F4E15E.3020109@redhat.com> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit Subject: Re: [Qemu-devel] migration question: disk images on nfs server List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: Orit Wasserman , Alexey Kardashevskiy , qemu-devel@nongnu.org On 07.02.2014 14:36, Orit Wasserman wrote: >> Do you know if is applies to linux O_DIRECT writes as well? >> > > From the man of open: > > The behaviour of O_DIRECT with NFS will differ from local > filesystems. Older kernels, or kernels configured in certain ways, > may not support this combination. The NFS protocol does not > support > passing the flag to the server, so O_DIRECT I/O will bypass the > page > cache only on the client; the server may still cache the I/O. The > client asks the server to make the I/O synchronous to preserve the > synchronous semantics of O_DIRECT. Some servers will perform > poorly > under these circumstances, especially if the I/O size is small. > Some > servers may also be configured to lie to clients about the I/O > having > reached stable storage; this will avoid the performance penalty at > some risk to data integrity in the event of server power failure. > The Linux NFS client places no alignment restrictions on O_DIRECT > I/O. > > To summaries it depends on your kernel (NFS client). So, assuming new kernel (where nfs O_DIRECT translates to no cache at client side) and cache coherent server, is it enough or is 'sync' mount (or O_SYNC flag) still required for some reason? -- mg