Strategy: Order Two Mediums Instead of Two Smalls and the EC2 Buffet

Vaibhav Puranik in Web  serving in the cloud – our experiences with nginx and instance sizes describes their experience trying to maximum traffic and minimum their web serving costs on EC2. Initially they tested with two m1.small instance types and then they the switched to two c1.mediums instance types. The m1s are the standard instance types and the c1s are the high CPU instance types. Obviously the mediums have greater capability, but the cost difference was interesting:

  • In the long term they will save money using the larger instances and not autoscaling. With the small instances, traffic bursts caused autoscaling to kick in. New instances were started in response to load. The instances woud be up for a short period of time and then spin down again. This constant churn costs a lot of money. Selecting the larger instance sizes, which are capable of handling the load without autoscaling, turn out to save money even though they are more expensive. Starting new instances also takes a few minutes  and they don't want to lose that traffic.

This went against my intuition, so I thought it might be a useful test for others to try.