Say you want to upgrade your laptop's RAM, what's the first thing you do if you have a new RAM stick in your hand? Turn off the laptop, install the new RAM, and restart. Pretty easy? Now imagine you're a system engineer at Amazon AWS. You are asked to upgrade the RAM of a physical machine, but it got some virtual machines running on it, what would you do? If you stop the server even for a second, the users will notice immediately. How to upgrade the RAM without shutting down the services?
The Zero-Downtime Dilemma
One thing you can do is to migrate the processes running on those virtual machines to a new ones. But the problem is the old virtual machine may still be needed for some network and system calls. Thus you can't shutdown the old machine easily.
What if you could take a virtual machine from one physical host and “teleport” it to another—without anyone noticing?
That’s not magic. It’s called live migration. With live migration, the users won't be having any issues (nor will they even know) if you replace the host of their virtual world. And on the other hand system engineers will have least time worrying about long migration time.
