During the last Data Center webinar I got an interesting question when describing the inherent problems of long-distance vMotion: “OK, I understand all the implications, but how do I persuade my server admins?”
The best answer I’ve heard so far came from an old battle-hardened networking guru: “Well, let them try”.
A bit more background: he’s running a really tight shop, with no inter-DC bridging. The application and server people in his company got infected with the anywhere-anytime “vision” and wanted to move live virtual machines between data centers. He decided to give them the tools they needed (read: more than enough rope).
Unintentionally they made the experiment more interesting by moving a virtual machine really far away from its data and all the other servers it was communicating with (for our purpose, the farther the better) ... and failed spectacularly. After a while, they came back saying “Now we understand what you were trying to tell us ... can we sit down and figure out what we can do?”
You don’t have to enable inter-DC vMotion to make them understand the problems. Just use a delay emulator like WANem and traffic shaping (to limit the bandwidth) and do the test in your lab.
You might find the following steps handy when replicating this feat:
- Understand the application traffic flows. The meshier the structure, the better.
- Document in advance what you expect to happen. Try to educate the server admins and the application people. If nothing else, make them aware why you’re opposed to inter-DC vMotion (it’s not because you’re lazy or because you don’t have a clue how to implement it).
- Make your boss (or CIO) aware of your predictions. You can’t say “I told them so” afterwards if your boss is not aware of the issue.
- Make sure the VM is moved under real load. The inter-DC link has to be well saturated before the traffic trombone data starts winding around the network.
- Enjoy the show ;)