Results: Random Performance
We turn to Iometer as our synthetic metric of choice for testing 4 KB random performance. Technically, "random" translates to a consecutive access that occurs more than one sector away. On a mechanical hard disk, this can lead to significant latencies that hammer performance. Spinning media simply handles sequential accesses much better than random ones, since the heads don't have to be physically repositioned. With SSDs, the random/sequential access distinction is much less relevant. Data are put wherever the controller wants it, so the idea that the operating system sees one piece of information next to another is mostly just an illusion.
4 KB Random Reads
Testing the performance of SSDs often emphasizes 4 KB random reads, and for good reason. Most system accesses are both small and random. Moreover, read performance is arguably more important than writes when you're talking about typical client workloads.
Plextor's M6S aces the other three drives with 32 outstanding commands. Transcend falls notably short at 68,000 IOPS. But when the queue drops to levels more typical of desktop PCs, the field tightens quite a bit. That's where you should focus your attention; most drives are so fast that commands hitting the controllers are serviced before they're able to back up.
4 KB Random Writes
Random write performance is also important. Early SSDs didn't do well in this discipline, seizing up even in light workloads. Newer SSDs wield more than 100x the performance of drives from 2007, though we also recognize that there's a point of diminishing returns in desktop environments.
The two SSDs with four-channel controllers succumb to the two drives with eight-channel Marvell processors between queue depths of two and four. Eventually, the M6S reasserts itself thanks to a capable Toggle-mode flash interface. But the SSD340 taps out at a queue depth of four.
Given the performance levels demonstrated by all four drives at a queue depth of one, though, it'll be interesting to see how they fare in our trace-based workloads.
Here's a break-down of the maximum observed 4 KB sequential read and write performance with Iometer. The order the drives appear in our chart is determined by maximum combined read and write performance.
