Am 13.09.2010 15:42, schrieb Anthony Liguori: > On 09/13/2010 08:39 AM, Kevin Wolf wrote: >>> Yeah, one of the key design points of live migration is to minimize the >>> number of failure scenarios where you lose a VM. If someone typed the >>> wrong command line or shared storage hasn't been mounted yet and we >>> delay failure until live migration is in the critical path, that would >>> be terribly unfortunate. >>> >> We would catch most of them if we try to open the image when migration >> starts and immediately close it again until migration is (almost) >> completed, so that no other code can possibly use it before the source >> has really closed it. >> > > I think the only real advantage is that we fix NFS migration, right?
That's the one that we know about, yes. The rest is not a specific scenario, but a strong feeling that having an image opened twice at the same time feels dangerous. As soon as an open/close sequence writes to the image for some format, we probably have a bug. For example, what about this mounted flag that you were discussing for QED? > But if we do invalidate_cache() as you suggested with a close/open of > the qcow2 layer, and also acquire and release a lock in the file layer > by propagating the invalidate_cache(), that should work robustly with NFS. > > I think that's a simpler change. Do you see additional advantages to > delaying the open? Just that it makes it very obvious if a device model is doing bad things and accessing the image before it should. The difference is a failed request vs. silently corrupted data. Kevin
