Mysql on SSD benchmarked ...
Roger Bitar did some really interesting benchmarks with mysql on SSD. In his benchmarks he artificially constrained the availability of memory for the mysql buffers (aka cache):
We choose to test MySQL, the open source database (DB), using a simple MySQL benchmark called Sysbench. We populated the Sysbench table with 112 Million rows (around 26GB size) that fit on 1 SSD drive . We executed read-only queries while varying the buffer size. We mounted the file system in DIRECTIO mode to disable file system caching. We perfrormed the tests with regular HDDs and repeated them with SSDs.
There results were somewhat astounding. Of course a fully memory saturated database is the fastest way to implement a database (buffer cache at 24 GB). But with a database on SSD Roger was able to yield 96 percent of the performance of the in-memory database even with heavily constrained RAM space (buffer cache limited to 8 GB). The latency at memory constrained situation is vastly better with SSD than with rotating rust. This leads me to an interesting conclusion: When you just compare the power consumption of SSD and hard disks (aka rotating rust), you forget a part of the equation. Let´s assume a FB DIMM takes 10 watts (it´s a little bit more, but 10 is easier for calculating). In the benchmark you were able to have almost the same performance with SSD (2 watts) at 8 GB as at 24 GB. Let´s assume 4 GB DIMMS. You would save 18 Watts (10*2-2) by using the SSD. Okay 18 watts isn´t a big deal, but now think about a 240 GB database. You can design your database server in quite a different manner, when you take SSD into your considerations.