Can I vertically scale an Amazon instance?
It is possible? According to Amazon,
Amazon EC2 Auto Scaling uses Horizontal scaling:
You can also use Amazon EC2 Auto Scaling for dynamic scaling of EC2 instances in order to automatically increase the number of Amazon EC2 instances during demand spikes to maintain performance and decrease capacity during lulls to reduce costs.
I found no reference to vertical scaling, by changing each instance specifications instead of adding or removing them.
To vertically scale an individual AWS EC2 instance up or down the instance has to be stopped, then the instance size changed, then restarted. This is due to the fact that Amazon has to often move the VM to a different piece of hardware with the available resources for the size change. Also, operating systems and software don't generally handle the sudden appearance of extra RAM or CPU cores very well.
Amazon does not provide a service for automatically scaling instances vertically, only horizontally, because horizontal scaling is the only type of scaling that doesn't incur down time.