From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from [140.186.70.92] (port=41093 helo=eggs.gnu.org) by lists.gnu.org with esmtp (Exim 4.43) id 1OHGy9-0000io-Jd for qemu-devel@nongnu.org; Wed, 26 May 2010 09:48:26 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.69) (envelope-from ) id 1OHGy6-0008Q6-Bv for qemu-devel@nongnu.org; Wed, 26 May 2010 09:48:25 -0400 Received: from mail-gw0-f45.google.com ([74.125.83.45]:44565) by eggs.gnu.org with esmtp (Exim 4.69) (envelope-from ) id 1OHGy6-0008Pb-8L for qemu-devel@nongnu.org; Wed, 26 May 2010 09:48:22 -0400 Received: by gwb11 with SMTP id 11so960152gwb.4 for ; Wed, 26 May 2010 06:48:20 -0700 (PDT) Message-ID: <4BFD269E.9090503@codemonkey.ws> Date: Wed, 26 May 2010 08:48:14 -0500 From: Anthony Liguori MIME-Version: 1.0 Subject: Re: [Qemu-devel] Re: [PATCH] Add cache=volatile parameter to -drive References: <1274091292-4812-1-git-send-email-agraf@suse.de> <4BF14CE9.5040907@suse.de> <4BF15DC8.8080104@codemonkey.ws> <201005171723.15675.paul@codesourcery.com> <4BF16E22.6090400@codemonkey.ws> <4BFC0FF6.1080005@suse.de> <20100525210113.GE1402@hall.aurel32.net> <4BFC79E8.1070700@codemonkey.ws> <20100526085248.GC21913@volta.aurel32.net> In-Reply-To: <20100526085248.GC21913@volta.aurel32.net> Content-Type: text/plain; charset=ISO-8859-15; format=flowed Content-Transfer-Encoding: 7bit List-Id: qemu-devel.nongnu.org List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: Aurelien Jarno Cc: kwolf@redhat.com, Paul Brook , qemu-devel@nongnu.org, Alexander Graf , hch@lst.de On 05/26/2010 03:52 AM, Aurelien Jarno wrote: > On Tue, May 25, 2010 at 08:31:20PM -0500, Anthony Liguori wrote: > >> On 05/25/2010 04:01 PM, Aurelien Jarno wrote: >> >>> I really think this patch can be useful, in my own case when testing >>> debian-installer (I already cache=writeback). In short all that is about >>> developing and testing, as opposed to run a VM in production, can >>> benefit about that. This was one of the original use case of QEMU before >>> KVM arrived. >>> >>> Unless someone can convince me not to do it, I seriously considering >>> applying this patch. >>> >> There really needs to be an indication in the --help output of what >> the ramifications of this option are, in the very least. It should >> > That's indeed something than can be done, but to avoid double standards, > it should also be done for other features that can lead to data > corruption. I am talking for example on the qcow format, which is not > really supported anymore. > I agree. >> also be removable via a ./configure option because no sane >> distribution should enable this for end users. >> >> > I totally disagree. All the examples I have given apply to qemu *users*, > not qemu developers. They surely don't want to recompile qemu for their > usage. Note also that what is proposed in this patch was the default not > too long ago, and that a lot of users complained about the new default > for their usage, they see it as a regression. We even had to put a note > explaining that in the Debian package to avoid to many bug reports. > cache=writeback only answer partially to this use case. > It's hard for me to consider this a performance regression because ultimately, you're getting greater than bare metal performance (because of extremely aggressive caching). It might be a regression from the previous performance, but that was at the cost of safety. We might get 100 bug reports about this "regression" but they concern much less than 1 bug report of image corruption because of power failure/host crash. A reputation of being unsafe is very difficult to get rid of and is something that I hear concerns about frequently. I'm not suggesting that the compile option should be disabled by default upstream. But the option should be there for distributions because I hope that any enterprise distro disables it. Regards, Anthony Liguori