From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([140.186.70.92]:47513) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1RJAHc-0003U0-Dc for qemu-devel@nongnu.org; Wed, 26 Oct 2011 16:41:09 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1RJAHa-0005RX-Ph for qemu-devel@nongnu.org; Wed, 26 Oct 2011 16:41:08 -0400 Received: from mail-iy0-f173.google.com ([209.85.210.173]:47189) by eggs.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1RJAHa-0005RS-MN for qemu-devel@nongnu.org; Wed, 26 Oct 2011 16:41:06 -0400 Received: by iakl21 with SMTP id l21so2644444iak.4 for ; Wed, 26 Oct 2011 13:41:05 -0700 (PDT) Message-ID: <4EA8705D.601@codemonkey.ws> Date: Wed, 26 Oct 2011 15:41:01 -0500 From: Anthony Liguori MIME-Version: 1.0 References: <4EA6ACFE.6090109@redhat.com> <4EA6B41B.3000903@codemonkey.ws> <4EA6C00B.3030701@redhat.com> <4EA6C25C.8000502@codemonkey.ws> <4EA6D676.2060907@redhat.com> In-Reply-To: <4EA6D676.2060907@redhat.com> Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 7bit Subject: Re: [Qemu-devel] KVM call agenda for October 25 List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: Kevin Wolf Cc: Paolo Bonzini , qemu-devel@nongnu.org, kvm@vger.kernel.org, Alexander Graf On 10/25/2011 10:32 AM, Kevin Wolf wrote: > Am 25.10.2011 16:06, schrieb Anthony Liguori: >> On 10/25/2011 08:56 AM, Kevin Wolf wrote: >>> Am 25.10.2011 15:05, schrieb Anthony Liguori: >>>> On 10/25/2011 07:35 AM, Kevin Wolf wrote: >>>>> Am 24.10.2011 13:35, schrieb Paolo Bonzini: >>>>>> On 10/24/2011 01:04 PM, Juan Quintela wrote: >>>>>>> >>>>>>> Hi >>>>>>> >>>>>>> Please send in any agenda items you are interested in covering. >>>>>> >>>>>> - What's left to merge for 1.0. >>>>> >>>>> I would still like to cache the default cache mode (probably to >>>>> cache=writeback). We don't allow guests to toggle WCE yet which Anthony >>>>> would have liked to see before doing the change. Is it a strict requirement? >>>> >>>> I don't see a way around it. If the default mode is cache=writeback, then we're >>>> open to data corruption in any guest where barrier=0. With guest togglable WCE, >>>> it ends up being a guest configuration issue so we can more or less defer >>>> responsibility. >>> >>> So do you think that offering a WCE inside the guest would be a real >>> solution or just a way to have an excuse? >> >> No, it offers a mechanism to "fix mistakes" at run-time verses at start up time. > > This is true (in both directions). But I think it's independent from the > right default. > >> It also means that you can make template images that understand that they >> don't support barriers and change the WCE setting appropriately. > > Isn't that really a job for management tools? > >>> Christoph said that OSes don't usually change this by themselves, it >>> would need an administrator manually changing the setting. But if we >>> require that, we can just as well require that the administrator set >>> cache=writethrough on the qemu command line. >> >> The administrator of the guest != the administrator of the host. > > But the administrator of the guest == the owner of the qemu instance, > no? He should be the one to use the management tools and configure his VMs. You're really talking about a multi-tenancy virtualization management solution. There really aren't a lot of these today. The most common variant is a IaaS platform where the end-user API is mostly just create a VM, destroy a VM. There's not a lot of dynamic configurability (just look at EC2s API). >>>> Do you think it's a good idea to change the default mode w/o guest WCE toggle >>>> support? What's your view about older guests if we change the default mode? >>>> What's your main motivation for wanting to change the default mode? >>> >>> Because people are constantly complaining about the awful >>> (cache=writethrough) performance they get before they are told they >>> should use a different cache option. And they are right. The >>> out-of-the-box experience with qemu's block performance really sucks. >> >> With qcow2 you mean, right? > > No, with any format, including raw. Which isn't surprising at all, > O_SYNC makes writes very expensive. > >>>> I'd be much more open to changing the default mode to cache=none FWIW since the >>>> risk of data loss there is much, much lower. >>> >>> I think people said that they'd rather not have cache=none as default >>> because O_DIRECT doesn't work everywhere. >> >> Where doesn't it work these days? I know it doesn't work on tmpfs. I know it >> works on ext[234], btrfs, nfs. > > I think tmpfs was named (and failing to start with default settings on > tmpfs would be nasty enough), but iirc Alex had another one. Alex? We can detect tmpfs with fsstat and do the right thing. Regards, Anthony Liguori > > Kevin >