As much as I like to site on my sofa or in my office chair and preach about the benefits of solid state drives, there are two issues that are difficult to overlook. SSDs are expensive when compared to traditional mechanical drives and they can't offer the same high capacity points either. The category for this is price vs. capacity and in this category SSDs have a poor track record. On the other hand, many users could care less about the high cost and lower capacity; the speed of SSDs is intoxicating and a bit addictive once you've taken your first dose.
Over the last couple of years we've seen companies try and merge the two technologies together. These hybrid solutions have come in many shapes and sizes and for several different markets. One of the first to launch was from Adaptec with their MaxCache designed for enterprise environments. LSI soon followed suit and released CacheCade and after that we were greeted with products from SilverStone (the HDDBoost) and finally there was Intel's attempt with Rapid Storage Technology. All of these products were a little different, but they all relied on the same principle - take a mechanical drive (or RAID of drives on the enterprise products) and pair them with an off the shelf SSD.
The problem with doing a cache system this way is there is a lot of performance that the SSD needs to make up. Since the HDD is already much slower than the SSD, you have a very large margin between the two. If you are using a small SSD, your hot data (data that is being cache) is only a small amount; this is what we found on the Intel RST solution. The larger your hard drive, the larger your solid state drive needs to be. This is especially true if you run several different programs and even more so if the programs you run most often are larger, like Photoshop. The holy grail for a cache solution is a large chunk of cache, in this case an SSD. In order to maximize performance and make up for the low speed of the HDD, you need a very fast SSD to get the same effect of running your entire system off of a solid state drive.
Let's take a look at OCZ's solution.