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. 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
