From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([2001:4830:134:3::10]:34016) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1UsTYP-0003Eh-PS for qemu-devel@nongnu.org; Fri, 28 Jun 2013 03:57:14 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1UsTYO-0006zA-CI for qemu-devel@nongnu.org; Fri, 28 Jun 2013 03:57:13 -0400 Received: from mx1.redhat.com ([209.132.183.28]:39750) by eggs.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1UsTYO-0006z4-2q for qemu-devel@nongnu.org; Fri, 28 Jun 2013 03:57:12 -0400 Message-ID: <51CD41CB.50303@redhat.com> Date: Fri, 28 Jun 2013 09:56:59 +0200 From: Paolo Bonzini MIME-Version: 1.0 References: In-Reply-To: Content-Type: text/plain; charset=ISO-8859-1 Content-Transfer-Encoding: 7bit Subject: Re: [Qemu-devel] [PATCH v2] xen_disk: support "direct-io-safe" backend option List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: Stefano Stabellini Cc: xen-devel@lists.xensource.com, Ian Jackson , qemu-devel@nongnu.org, Anthony Liguori Il 27/06/2013 20:16, Stefano Stabellini ha scritto: > Support backend option "direct-io-safe". This is documented as > follows in the Xen backend specification: > > * direct-io-safe > * Values: 0/1 (boolean) > * Default Value: 0 > * > * The underlying storage is not affected by the direct IO memory > * lifetime bug. See: > * http://lists.xen.org/archives/html/xen-devel/2012-12/msg01154.html > * > * Therefore this option gives the backend permission to use > * O_DIRECT, notwithstanding that bug. > * > * That is, if this option is enabled, use of O_DIRECT is safe, > * in circumstances where we would normally have avoided it as a > * workaround for that bug. This option is not relevant for all > * backends, and even not necessarily supported for those for > * which it is relevant. A backend which knows that it is not > * affected by the bug can ignore this option. > * > * This option doesn't require a backend to use O_DIRECT, so it > * should not be used to try to control the caching behaviour. > > Also, BDRV_O_NATIVE_AIO is ignored if BDRV_O_NOCACHE, so clarify the > default flags passed to the qemu block layer. > > The original proposal for a "cache" backend option has been dropped > because it was believed too wide, especially considering that at the > moment the backend doesn't have a way to tell the toolstack that it is > capable of supporting it. Given how rusty my xenstore-fu is, I'll ask the obvious: the frontend cannot write to it, can it? Paolo From mboxrd@z Thu Jan 1 00:00:00 1970 From: Paolo Bonzini Subject: Re: [PATCH v2] xen_disk: support "direct-io-safe" backend option Date: Fri, 28 Jun 2013 09:56:59 +0200 Message-ID: <51CD41CB.50303@redhat.com> References: Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1 Content-Transfer-Encoding: 7bit Return-path: In-Reply-To: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Errors-To: qemu-devel-bounces+gceq-qemu-devel=gmane.org@nongnu.org Sender: qemu-devel-bounces+gceq-qemu-devel=gmane.org@nongnu.org To: Stefano Stabellini Cc: xen-devel@lists.xensource.com, Ian Jackson , qemu-devel@nongnu.org, Anthony Liguori List-Id: xen-devel@lists.xenproject.org Il 27/06/2013 20:16, Stefano Stabellini ha scritto: > Support backend option "direct-io-safe". This is documented as > follows in the Xen backend specification: > > * direct-io-safe > * Values: 0/1 (boolean) > * Default Value: 0 > * > * The underlying storage is not affected by the direct IO memory > * lifetime bug. See: > * http://lists.xen.org/archives/html/xen-devel/2012-12/msg01154.html > * > * Therefore this option gives the backend permission to use > * O_DIRECT, notwithstanding that bug. > * > * That is, if this option is enabled, use of O_DIRECT is safe, > * in circumstances where we would normally have avoided it as a > * workaround for that bug. This option is not relevant for all > * backends, and even not necessarily supported for those for > * which it is relevant. A backend which knows that it is not > * affected by the bug can ignore this option. > * > * This option doesn't require a backend to use O_DIRECT, so it > * should not be used to try to control the caching behaviour. > > Also, BDRV_O_NATIVE_AIO is ignored if BDRV_O_NOCACHE, so clarify the > default flags passed to the qemu block layer. > > The original proposal for a "cache" backend option has been dropped > because it was believed too wide, especially considering that at the > moment the backend doesn't have a way to tell the toolstack that it is > capable of supporting it. Given how rusty my xenstore-fu is, I'll ask the obvious: the frontend cannot write to it, can it? Paolo