I've been evaluating Amazon Web Services RDS for MySQL and costing out potential scenarios involving a simple multi-AZ deployment read/write setup vs. a multi-AZ deployment mysql master (hot-standby) with additional read-only slaves. the issue I'm trying to cost-optimize includes their reserved instance vs on-demand instances.

Situation 1: purchase reserved multi-az setup for Extra-large-hi-mem(17GB RAM) instance for $5200/yr and have my application query the master all the time. the problem is, if I don't need all the resources of the (17GB RAM) all the time and therefore, especially not a hot-standby, what alternatives for savings can a better topology create, like potentially situation 2 below:

Situation 2: purchase reserved multi-az setup using smaller master instances than above for the master-master hot-standby to receive the writes only. Then create and load balance several read-only slaves off the master and add/remove and/or scale up/down the read slaves based on demand. This might only cost $1000 + the on-demand usage of the read slaves.

My thinking is, if I have a variable read-intensive application load, with low write load, the single level topology in situation 1 means I'm paying for a lot of resources at the write level of topology when I don't need them there. My hope is that situation 2 can yield cost savings from smaller reserved instances on the master-master resource level allowing me to scale up and down and/or out on the read-level according to demand as needed.

Does anyone see a downside to doing this or know of some reason this isn't possible with RDS? Any other thoughts or advice always welcome of course.

Thanks in advance,

R