On Mon, Sep 13, 2010 at 3:13 PM, Kevin Wolf <kw...@redhat.com> wrote: > 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?
There is some room left to work in, even if we can't check in open(). One idea would be to do the check asynchronously once I/O begins. It is actually easy to check L1/L2 tables as they are loaded. The only barrier relationship between I/O and checking is that an allocating write (which will need to update L1/L2 tables) is only allowed after check completes. Otherwise reads and non-allocating writes may proceed while the image is not yet fully checked. We can detect when a table element is an invalid offset and discard it. Stefan
