On 25.10.2011, at 17:32, Kevin Wolf <kwolf@xxxxxxxxxx> 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. > >>>> 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. Yeah, IIRC NFS also failed on me :) Alex > > Kevin -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
