Results: 4 KB Random Writes
Random write performance is extremely important, no question about it. Early SSDs didn't do well in this metric at all, seizing up in even the most lightweight workloads. Newer SSDs wield more than 100x the performance of drives from 2007, but there's a point of diminishing returns in client environments. When you swap a hard drive out for solid-state storage, your experience improves. Load times, boot times, and system responsiveness all get better. If called upon, your SSD-equipped system could handle a lot more I/O than the spinning media you had in there before. With consumer workloads, it's more about getting to those operations faster, not necessarily handling more of them.
4 KB Random Write
Shining a spotlight on each M500 capacity point gives us a clear look at the impact of parallelism in solid-state storage. Write speed is very much a function of how many dies there are to exploit. We already saw this playing out in the sequential write test, and it's even more pronounced when we write small blocks of data randomly.
Crucial's 120 GB M500 passes 34,000 IOPS at a queue depth of two. The 240 GB drive doubles that number. Both larger M500s continue scaling, but match each others' performance as they plateau. When you're down at a queue depth of one, though, all four SSDs post the same ~30,000 IOPS finish.
Clearly, the 120 GB M500 can't hang with the big dogs, though it does rub up against the 256 GB Ultra Plus. That one model sports the only four-channel controller on our chart, and the impact is painfully obvious.
Crucial's two highest-capacity M500s give performances worthy of front-runner status. The 240 GB version is more modest. Athough it does a lot better than the 120 GB M500, transitioning to 128 Gb flash hurts the benchmark results in a measurable way.
Write Saturation
A saturation test consists of writing to a drive for a specific period of time with a defined workload. Technically, this is a fairly enterprise-class write saturation test, where the entire LBA space of the disk is utilized.
In Iometer and in this write saturation test, all four SSDs start out secure-erased. In the write saturation testing, however, where each drive is peppered across its entire LBA range with 4 KB random writes at a queue depth of 32, the larger M500s start by falling short of the big numbers we saw in Iometer (the were both up above 80,000 IOPS previously).
It's possible that both drives have to adjust to a larger LBA range. But if that's the case, then why don't the smaller models exhibit the same behavior? This could be an artifact of the larger drives' higher performance, whereas the 120 and 240 GB M500s are limited to 35,000 and 60,000 IOPS, respectively.
Given that the 480 and 960 GB models are so similar in terms of speed, it's easy to see that the larger SSD takes roughly twice as long to fill up. With the move from 120 to 240 to 480 GB, capacity jumps in proportion to write performance. That's why the 240 GB M500, despite its substantial advantage in IOPS, takes the same amount of time as the 120 GB drive to drop to a lower performance level on its way to steady-state.
