Conclusions
A great many things have changed in the storage world in the last four and a half years. We've mainly focused on performance today, but I suspect the rapid rise of mechanical hard drive capacity has had the greatest impact on users. SSDs remain an expensive proposition even for enthusiasts, while just about anyone can afford a sub-$100 terabyte desktop drive or 500GB mobile unit. The fact that it's taken less than five years for hard drive capacities to grow by a factor of eight is a testament to just how effective perpendicular recording technology has been at increasing areal densities.

Although the capacities of mechanical hard drives have snowballed, the performance gains have been considerably more modest. Higher areal densities have improved transfer rates, both in real-world file operations and in synthetic tests, but we're looking at roughly a two-fold increase in performance, at best. Mechanical and rotational latencies are even tougher problems to solve, and we've seen little progress on this front. The Raptors have enjoyed success at 10k RPM, but the vast majority of desktop drives have remained at 7,200 RPM. Some, like Western Digital's Caviar Green series, have even lowered spindle speeds in a bid to cut power consumption and noise levels.

Lower spindle speeds are common among 2.5" notebook hard drives, but the latest 7,200-RPM models give older desktop units a run for their money. Even if you don't want to shell out for an SSD, swapping in a fast 7,200-RPM hard drive is usually the best way to improve notebook performance, particularly with budget models that are typically equipped with older 5,400-RPM drives—or, shudder, a 4,200-RPM clunker.

SSDs pose perhaps the biggest threat to notebook hard drives, in part because they share the same 2.5" form factor. SSDs tend to have higher shock tolerance and lower power consumption, too, and the highest capacity notebook hard drives top out at only half a terabyte.

If you look at our performance results, though, today's SSDs seem best suited to the sort of random access patterns typical of multitasking and multi-user environments, such as high-end workstations and servers of various types. Solid-state drives may be expensive, but if you consider their performance per dollar, some offer phenomenal value for enterprise applications. Then again, we've not tested the 15k-RPM SAS and SCSI drives against which SSDs must to compete in those markets.

SSDs have long had a substantial seek time advantage over their mechanical counterparts. Only recently has silicon-based storage also enjoyed faster sequential transfer rates. Solid-state drives are still relatively new, with lots of room for further performance growth. Mechanical hard drive technology is more mature, and while capacities continue their rapid ascent, the performance improvements seem stunted by comparison.

As solid-state drive prices continue to fall and mechanical hard drive capacities continue to climb, I suspect we'll see the two technologies coexist in most enthusiast desktop systems. Even today, there's a good case to be made for running an SSD as an operating system and applications drive backed by a terabyte or two of mechanical storage. That might not be the dominant configuration a few years from now, but based on how rapidly SSDs are evolving, it'll probably be the best one.