I run a database cluster with dozens of nodes on EC2. Small entries, lots of small IOPS.

From http://orion.heroku.com/past/2009/7/29/io_performance_on_ebs..., "On a good day, an EBS can give you 7,000 seeks per second and on a not so good day will give you only 200."

The ephemeral store will never give you more than a hundred seeks per second. If you're seek-bound, then EBS, every time.

My experience has been that EBS handles concurrent loads better, ephemeral drives handle non-concurrent loads better.

Transferring 100gb+ of data on EBS (even with an 8x RAID) is a nightmare. Ephemeral drives, however, it's fairly fast.

Throw 100+ database connections at a few ephemeral drives (even in a RAID) and watch your web site slow to a crawl.