Q:

AWS: Should my EC2 and RDS instances be in the same Availability Zone?

I just noticed that all of our EC2 instances are in zone us-west-2b, but our Multi-AZ RDS instance is in us-west-2a.
Performance-wise everything seems to be okay, and it will be a hassle to "move" the instances to one place since you have to stop and re-create them all. However if either of the two zones goes down when we will have some downtime; if everything is in one zone then at least we have a higher chance of not being in the zone that has downtime...
Is this something worth fixing, or am I over-thinking it?
(I was about to purchase some EC2 Reserved Instances, which are tied to specific AZs, so I wanted to make sure before going through with it)
Thanks!

A:

The Multi-AZ nature of your RDS means it may not always be in that AZ - they can move around. In the event of a failure of its current primary AZ, it'll fail over into another one automatically. As such, it doesn't make sense to move EC2 instances around to follow it when it could move away on its own.
You should consider, however, moving some of your EC2 instances into another AZ (maybe from us-west-2b to us-west-2a) so that if an AZ goes down you'll have both the RDS and some of your EC2 instances still up and running.

