From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from [140.186.70.92] (port=39378 helo=eggs.gnu.org) by lists.gnu.org with esmtp (Exim 4.43) id 1PuPVo-00081y-Om for qemu-devel@nongnu.org; Tue, 01 Mar 2011 08:21:14 -0500 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1PuPVn-00058Y-ER for qemu-devel@nongnu.org; Tue, 01 Mar 2011 08:21:12 -0500 Received: from mx1.redhat.com ([209.132.183.28]:34015) by eggs.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1PuPVm-00058B-OH for qemu-devel@nongnu.org; Tue, 01 Mar 2011 08:21:11 -0500 Message-ID: <4D6CF32C.4090709@redhat.com> Date: Tue, 01 Mar 2011 14:22:52 +0100 From: Kevin Wolf MIME-Version: 1.0 Subject: Re: [Qemu-devel] [RFC][PATCH 0/2] Allow cache settings for block devices to be changed at runtime. References: <20110228171956.05a84fb9@zephyr> <4D6BBB72.6040205@redhat.com> <4D6BC3DE.7000208@redhat.com> In-Reply-To: Content-Type: text/plain; charset=ISO-8859-1 Content-Transfer-Encoding: 7bit List-Id: qemu-devel.nongnu.org List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: Anthony Liguori Cc: Anthony Liguori , Stefan Hajnoczi , Stefan Hajnoczi , qemu-devel , Ananth Narayan , Prerna Saxena , Christoph Hellwig Am 01.03.2011 14:03, schrieb Anthony Liguori: > > On Feb 28, 2011 10:48 AM, "Kevin Wolf" > wrote: >> >> Am 28.02.2011 16:35, schrieb Stefan Hajnoczi: >> > On Mon, Feb 28, 2011 at 3:12 PM, Kevin Wolf > wrote: >> >> Am 28.02.2011 12:49, schrieb Prerna Saxena: >> >>> The following patchset introduces monitor commands: >> >>> >> >>> 1. set_cache DEVICE CACHE-SETTING >> >>> Change cache settings for block device, DEVICE, through the monitor. >> >>> (Available options : 'none', 'writeback', 'writethrough') >> >>> Eg, >> >>> (qemu)set_cache ide0-hd0 none >> >>> -> Changes cache setting for ide0-hd0 to 'none' >> >> >> >> Not sure if adding this interface is a good idea. I see that you only >> >> add it for HMP, and we may consider that, but it's definitely not >> >> suitable for QMP. >> >> >> >> One reason is that none/writethrough/writeback/unsafe isn't really what >> >> we want to use long term. We want to separate advertising a write cache >> >> (which is guest visible) from things like whether to use O_DIRECT > or not. >> >> >> >> In the past, Christoph mentioned that he had patches to make these >> >> separate and even let the guest change the "write cache enabled" flag, >> >> which would probably solve most of the use cases of this patch. >> > >> > Toggling host page cache at runtime is useful too because it saves >> > having to restart VMs. >> >> Not sure why I wanted to change that during runtime, but agreed, >> allowing to change parameters using the monitor is generally a good thing. >> >> However, I'm not sure if a command for changing the cache mode is the >> right solution, or if it should be something like a command to change >> block device options. (For example, what about toggling read-only or >> snapshot mode?) > > Certainly good questions, but let me suggest not taking an HMP command > and not a QMP commans because of interface concerns. > > My goal for 0.15 is to convert HMP to be implemented in terms of QMP. > To do that, a bunch of new QMP commands are needed. They all won't be > perfect but i'd rather support a bad QMP command forever than to > continue to/ have people rely on HMP. Okay, makes sense. So we should reject patches that add new HMP commands without adding a QMP counterpart. >> > I agree that the guest should control the >> > emulated drive cache at runtime and we probably don't want to allow >> > toggling that from the host - it could be dangerous :). >> >> Good point. That's a NACK for this patch as long as we haven't separated >> WCE from the host cache setting. Doesn't make a difference for this one, though, because it's NACKed anyway. Kevin PS: Anthony, is there a specific reason why you started sending HTML emails?