From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from [140.186.70.92] (port=39135 helo=eggs.gnu.org) by lists.gnu.org with esmtp (Exim 4.43) id 1PuMIN-0004OO-QS for qemu-devel@nongnu.org; Tue, 01 Mar 2011 04:55:08 -0500 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1PuMIM-0003kP-Dt for qemu-devel@nongnu.org; Tue, 01 Mar 2011 04:55:07 -0500 Received: from mail-yw0-f45.google.com ([209.85.213.45]:53713) by eggs.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1PuMIM-0003kL-8M for qemu-devel@nongnu.org; Tue, 01 Mar 2011 04:55:06 -0500 Received: by ywl41 with SMTP id 41so1979881ywl.4 for ; Tue, 01 Mar 2011 01:55:05 -0800 (PST) MIME-Version: 1.0 In-Reply-To: <4D6BC3DE.7000208@redhat.com> References: <20110228171956.05a84fb9@zephyr> <4D6BBB72.6040205@redhat.com> <4D6BC3DE.7000208@redhat.com> Date: Tue, 1 Mar 2011 09:55:05 +0000 Message-ID: Subject: Re: [Qemu-devel] [RFC][PATCH 0/2] Allow cache settings for block devices to be changed at runtime. From: Stefan Hajnoczi Content-Type: text/plain; charset=ISO-8859-1 List-Id: qemu-devel.nongnu.org List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: Kevin Wolf Cc: Anthony Liguori , Stefan Hajnoczi , qemu-devel , Ananth Narayan , Prerna Saxena , Chunqiang Tang , Christoph Hellwig On Mon, Feb 28, 2011 at 3:48 PM, 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?) Yes, I think you're right. We should aim for a general interface rather than having to add many more specific interfaces in the future. CQ: Do you see a relation to the "update" interface you added to adjust drive options at runtime for FVD? Stefan