From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([140.186.70.92]:37165) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1QbuNg-0004K2-8P for Qemu-devel@nongnu.org; Wed, 29 Jun 2011 09:00:38 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1QbuNd-0002Bk-Rc for Qemu-devel@nongnu.org; Wed, 29 Jun 2011 09:00:36 -0400 Received: from mx1.redhat.com ([209.132.183.28]:31471) by eggs.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1QbuNd-0002Bb-95 for Qemu-devel@nongnu.org; Wed, 29 Jun 2011 09:00:33 -0400 Message-ID: <4E0B21E0.6030404@redhat.com> Date: Wed, 29 Jun 2011 16:00:16 +0300 From: Avi Kivity MIME-Version: 1.0 References: <4E0B1399.4050807@redhat.com> In-Reply-To: <4E0B1399.4050807@redhat.com> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit Subject: Re: [Qemu-devel] Default cache mode List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: Kevin Wolf Cc: Christoph Hellwig , Qemu-devel@nongnu.org, Stefan Hajnoczi On 06/29/2011 02:59 PM, Kevin Wolf wrote: > Hi, > > I think we have touched this topic before during some IRC discussions or > somewhere deep in a mailing list thread, but I think it hasn't been > discussed on the list. > > Our default cache mode of cache=writethrough is extremely conservative > and provides absolute safety at the cost of performance, and most people > don't use it if they know that it can be changed because it just > performs too bad. There are use cases where you need it (broken guest > OS), but none and writeback are just as correct with respect to the > specs and they are safe to use with current OSes. And even with broken > OSes, in many use cases it doesn't really matter if you lose a VM and > have to reinstall it (which is probably true even more for users > invoking qemu directly instead of using libvirt). > > I think the motivation to switch from writeback to writethrough as > default was that writeback was entirely unsafe back then. This isn't > true any more, so is there still enough reason to have the slow > writethrough mode as default? > > I'm not entirely sure if I should suggest writeback or none as the new > default, but I think it could make sense to change it. So long as -M old retains the old behaviour, I'm in favour. I think writeback is probably a better default than none. -- error compiling committee.c: too many arguments to function