Drupal scalability, Part 2

This is a follow up to a writeup I did last year. Read the previous article about Drupal scalability for the back story. In five months nearly nothing has changed in the production environment at work. What has changed is my understanding of disk performance, as it relates to databases, and the power of higher level caching.

I couple months ago I spent many hours looking at disk stats in Linux with the iostat utility. How many writes per second can a three disk RAID 5 array do?