Am 03.07.2012 15:51, schrieb Paolo Bonzini: > Il 03/07/2012 15:49, Kevin Wolf ha scritto: >>> If the guest does not support flushes, we should run in writethrough mode. >>>> The setting is temporary until the next reset, so that for example the >>>> BIOS will run in writethrough mode while Linux will run with a writeback >>>> cache. >>>> >>>> VIRTIO_BLK_F_FLUSH has been introduced in Linux 2.6.32 (in 2009) and >>>> was backported to RHEL/CentOS 5.6 (in 2010). The Windows drivers have >>>> two bugs, which I reported on the Red Hat Bugzilla as bugs 837321 and >>>> 837324. With these patches they will suffer a performance hit but >>>> gain correctness. >>>> >>>> Signed-off-by: Paolo Bonzini <pbonzini@xxxxxxxxxx> >> I generally like the idea for a default, but doesn't this override even >> an explicit cache=writeback? > > Yes. It doesn't override cache=unsafe though. When the guest doesn't support flushes, cache=writeback is equivalent to cache=unsafe, so if you want the old behaviour back you can switch to cache=unsafe without additional risks. We don't have a cache=directunsafe, though, so if you want to get the old behaviour of cache=none back, you're out of luck. Not sure how acceptable this is. Irrespective of this concern I've come to the conclusion that I agree and we actually must enforce this for non-unsafe mode, and not doing it is a bug. 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
