Making the Case for Long Distance Virtual Machine Mobility

With VMWorld coming up I’m reminded of a top of mind subject, virtual machine mobility. The reason for moving virtual machines is to better allocate server resources and maintain application performance. It’s a useful technology that works great in the data center. We also hear a lot about the need to move virtual machines across the WAN, live, without losing sessions. This is known as Long Distance vMotion or generically as long distance live migration. This might sound like a good idea, but it gets a bit complicated when you think outside the data center walls and across the WAN. It creates complexity in the network, as maintaining sessions requires keeping the same IP address and MAC address after the move.  There are many proposed use cases for it, but is it such a good idea?

Limitations of Live Migration
Long distance live migration over the WAN has limitations due to latency and bandwidth requirements and the complexity of extending layer 2 which is required to maintain the same MAC address. Issues include the potential for traffic coming first to the original data center where the gateway is, and then looping to the new data center where the VM has moved.  Traffic can also loop back over the WAN to reach the storage that stayed behind. There are also bandwidth requirements to handle the large scale movement, as well as issues with storage pooling and storage replication as well as the complexity of implementing the L2 bridging architecture. If we are going to deal with all of this complexity of moving virtual machines over the WAN there had better be a good reason to do it. But is there? Here is a look at the various use cases for long live migration that I have found.