Some more thoughts on disk IO and calculations….

David - The number of spindles you have backing your databases needs to be based on throughput (the IOs per second). You should not depend on WriteBack Cache to reduce the numbers of IO. Since the database writes are random, you can’t expect coalescing to significantly reduce the number of writes. The advantage of the write back is that writes to the cache return the OS immediately – this reduces the write latency, but does not increase the overall throughput. The data still needs to be written to disk. That means if you overburden it, it will just fill the cache because the disks won't be able to keep up. For normal IO on a healthy diskset, you should see writes around 10-15ms (factoring some queue delay, head seeking, and data transfer time). With WriteBack caching, you'll see 1-5 ms write times. In summary: write cache reduces the latencies on a well performing system, but does buy any reduction in the number of IOS.