[jstin]

Heroku sells convenience, and resells EC2 instances to you. Their previous dedicated database option, Ronin, cost $200/month. When you connect the dots that this DB has a 1.7GB cache and that a small instance on EC2 has 1.7GB memory[1], you start to realize what you're really paying for.

While paying 3.3 times more[2] may be ok for the added simplicity, you have to wonder if your 'dedicated' database is really just a small instance on EC2? If it is a larger instance, then you are sharing it with other users.

However, even a small instance is far from not being shared. EC2 has performance issues, especially small instances. Disk IO is the worst.[3]

Disclaimer: These are all just my observations, and I don't how Heroku actually configure their Ronin databases. I'd love to be proven wrong, and to have someone from Heroku explain. But from personal experience with both EC2 and Heroku's Ronin database, if my conclusions are wrong, the results are not. I've seen very slow performance on the simplest of queries on both configurations.

[1] http://aws.amazon.com/ec2/instance-types/ [2] http://aws.amazon.com/ec2/pricing/ [3] http://www.frederico-araujo.com/2011/12/27/why-ec2-still-sux...

[zokier]

I didn't think disk io matters all that much if you fit your data in memory, which afaik you should if you want any kind of decent performance.

[markatto]

Even if your whole data set fits into main memory, writes still have to go to disk.