ignazio.cassano@provincia.torino.it
2007-Jul-02 09:54 UTC
[Xen-users] xen 3.1 live migration
Good morning, I installed 3 fedora core 7 machines with xen 3.1 Two machine (hp dl 360) are identical and windows guest live migration works fine. One of them is a HP dl 580 and live migration from/to hp 360 does not work. Stopping and restarting guests works in any case also if hardware is non identical. I read somewhere machine must be identical for live migration: is it true ? Why live migration does not work also if cpu type is almost identical ? And why stopping and restarting guest works in any case ? Many thanks Igna _______________________________________________ Xen-users mailing list Xen-users@lists.xensource.com http://lists.xensource.com/xen-users
Hi, When Linux boots it tests what CPU features are available on this machine and may choose to make use of them. For instance, MMX, 3DNow, etc may be used by the kernel to accelerate certain operations if they are available. Also, userspace libraries may go through a similar initialisation process in order to choose an optimised implementation of a call. The problem is they don''t expect these features to change. Suppose you boot a domain on a CPU that has 3DNow, and Linux decides to use it. Then you suspend/resume or live migrate to a machine without 3DNow. Linux won''t be able to change the instructions it''s using, so when it invokes an instruction this CPU doesn''t support it''ll crash with an invalid opcode exception. If you shut down a machine and start it on different hardware then it''ll probe the CPU of the new hardware on bootup and start using features that that system supports. Migration between systems ought to be OK as long as Linux (and userspace) do not "lose" CPU features that they were using in the process. So e.g. migrating between two CPUs with the same features but different speeds or cache sizes should not crash the domain. Cheers, Mark -- Dave: Just a question. What use is a unicyle with no seat? And no pedals! Mark: To answer a question with a question: What use is a skateboard? Dave: Skateboards have wheels. Mark: My wheel has a wheel! _______________________________________________ Xen-users mailing list Xen-users@lists.xensource.com http://lists.xensource.com/xen-users