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 Read
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.
128/256 GB Adata SP920
Against the smaller M500s, Adata's SSDs establish a slim lead at a queue depth of one and lead from there. Low-queue depth random read performance is a very relevant measurement, and the SP920s manage to improve, even using (more or less) the same flash. Likely, the gains come from the updated controller and firmware instead.
Of course, none of these drives can come close to Samsung's 840 EVO, which is up to 25% quicker at a queue depth of one.
With 32 outstanding commands, Adata's 128 GB SP920 is on par with the 240 GB M500.
512/1024 GB Adata SP920
It comes as no surprise that the 512 and 1024 GB SP920s are functionally identical to Crucial's M550s at the same capacity points, even though were able to coax a few hundred more IOPS out of Crucial's drives. That's pretty standard though, and we might see the same variance from the same SSD on different runs.
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.
128/256 GB Adata SP920
Sporting fewer dies, these lower-capacity drives tap out earlier. Our workload is sufficiently intense to saturate the higher-density configurations at lower queue depths.
Still, we get 16,000 more IOPS from the 256 GB SP920 compared to the 240 GB M500, and 10,000 more IOPS from Adata's 128 GB model versus the 120 GB M500. The drives feature the same number of addressable NAND elements, so most of the speed-up, again, likely comes from the controller.
512/1024 GB Adata SP920
We hit more than 80,000 IOPS with just four outstanding commands. The M550 platform wrings out over 91,000 tested similarly. These drives plateau at a queue depth of eight, so latency is best there. Beyond that, every model offers the same performance, albeit at higher latency.
Random Performance Over Time
My saturation test consists of writing to each drive for a specific duration with a defined workload. Technically, it's an enterprise-class benchmark, where the entire LBA space of the SSD is utilized by a random write at high queue depths.
Here's 12 hours of a 4 KB write with 32 outstanding commands. First, we secure erase each drive. Then we apply the 4 KB write load, showing the average IOPS for each minute (except for the last 20 minutes, where we zoom in and show you one-second average increments).
After the first drive fill, performance drops off fast, since the SSD no longer has free blocks to write to. Instead, they have to be erased prior to subsequent writes.
Today we're showing a breakout of both drive families at steady state for a 4 KB write at a queue depth of 32.
I wanted to use this workload to show that the Crucial M550 and Adata SP920 drive families behave similarly. More so than just the raw performance numbers, we want to characterize these SSDs. And as I might have predicted, this test shows that two identically-configured devices (the 512 GB models, specifically), are more or less the same.
Here's a breakdown 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.
Drives like the 1024 GB SP920 are about as fast as the SATA 6Gb/s interface will let them be.
The overhead associated with small transfers brings down the throughput ceiling. That is, it's easy to saturate SATA with large sequential operations. But these drives demonstrate you can do somewhere around 400 MB/s with aligned 4 KB blocks.