Results: Tom's Storage Bench v 1.0, Continued
Service Times
Beyond the average data rate reported on the previous page, there's even more information we can collect from Tom's Hardware's Storage Bench. For instance, mean (average) service times show what responsiveness is like on an average I/O during the trace.
It would be difficult to graph the 10+ million I/Os that make up our test, so looking at the average time to service an I/O makes more sense. For a more nuanced idea of what's transpiring during the trace, we plot mean service times for reads against writes. That way, drives with better latency show up closer to the origin; lower numbers are better.
Write latency is simply the total time it takes an input or output operation to be issued by the host operating system, travel to the storage subsystem, commit to the storage device, and have the drive acknowledge the operation. Read latency is similar. The operating system asks the storage device for data stored in a certain location, the SSD reads that information, and then it's sent to the host. Modern computers are fast and SSDs are zippy, but there's still a significant amount of latency involved in a storage transaction.
Only SanDisk's A110 PCIe M.2 can boast better write service times, and no SSD is significantly better when it comes to mean read service times.
Isolating the range of M500s and M550s, Crucial's drives span the entire width and breadth of this chart's scale. The 120 GB M500 is off on its own, while the larger M500s get crammed somewhere in the middle. Both M550s end up in favorable real estate; south and west is where you want to be.
The M550s end up in the middle of this chart. The differences between our tested drives aren't enormous, though they do add up. Fortunately, the M550s beat Crucial's 480 and 960 GB M500.
Just look at the M550s go! The top two spots are a tie; the 512 and 1024 GB models are only rivaled by each other.
Our mean write service chart shows a greater range than the read chart did. The same observation can be made about the read-versus-write comparison at the top of this page. Vertically, it stretches on and on, reflecting a massive spread in write times.
The slowest drive is the 120 GB M500, with a mean service time of 2543 microseconds. At the other end, Crucial's M550s both report incredible 620-microsecond write service times. That means the 120 GB M500 takes four times longer, on average, to service each write operation in our trace. In reality, that's not such a noticeable wait. But the small M500 sure isn't winning any popularity contests. Micron knows this. As mentioned previously, the 128 GB M550 uses twice as many dies to improve parallelism. We expect it to rival a number of 256 GB competitors as a result.
