I haven't done the cost analysis lately, but what this does is bring more IOPS in at a lower price point. The cost of RAM is not static, the more you have the

I haven't done the cost analysis lately, but what this does is bring more IOPS in at a lower price point. The cost of RAM is not static, the more you have the more expensive it gets because you need better motherboards and specialized chassis to handle it and really manage all those buses. SSDs bring in an intermediate point between magnetic disks and RAM so that you can get some more IOPS in at that lower price point. Of course, if you need all you can get you should always go for just RAM, but you can get a significant improvement with a modest investment through SSDs. The best example would be the large data set (say 100GB with a random access pattern or 100GB of working set) with enough requests that it starting to strain the RAID system you have. At this point your medium is saturated so latency is going up and up. Getting 128 GB of RAM is expensive, but swapping those drives out for a few 100GB SSDs is not hard and may alleviate the DB bottle-neck until something else becomes the bottle-neck and you need to change that anyway. Switching to RAM wouldn't have bought you anything more than the SSDs because in both cases you are getting acceptable performance up to the next bottleneck. I'd like to think that SSDs will get us to the point that we don't have to worry about disk latency and can just toss data on there without having the database always be the bottleneck.