Results: 128 KB Sequential Performance
Fantastic sequential read and write performance is a trademark of modern SSDs. To measure it, we use incompressible data over a 16 GB LBA space, and then test at queue depths from one to 16. We're reporting these numbers in binary (where 1 KB equals 1024) instead of decimal numbers (where 1 KB is 1000 bytes). When necessary, we're also limiting the scale of the chart to enhance readability.
128 KB Sequential Read
Samsung's four mSATA-based 840 EVOs top 500 MB/s with just two outstanding commands. Read performance, despite three-bit-per-cell NAND, remains a strength. These are the kind of numbers we'd expect from the 840 Pro, but it's more surprising to see coming from the company's 840 and 840 EVO. Still, with sequential reads, we'd be concerned if performance wasn't this high.
128 KB Sequential Write
Sequential writes are where we see the benefit of more dies. As capacity shifts from 120 to 1000 GB, write performance increases substantially. Admittedly, Samsung's Turbo Write technology does affect the outcome by using some of the MLC as emulated single-level cell flash. And of course, the larger models have more of this faster space at their disposal. Because this is how we test, the results end up quite favorable for Samsung. But expect minimum performance (what you'll see once the cache limits are surpassed) to be a good deal lower.
Here's a break-down of the maximum observed 128 KB sequential read and write performance with Iometer:
The 840 EVO mSATA drives manage respectable finishes in our sequential performance testing. Demonstrating very similar read speeds, positioning in this chart comes down to the results of our write testing. Not only does write performance scale with the number of dies, but Samsung's Turbo Write cache size increases with capacity, too. That means the 1000 GB model with 12 GB of cache is able to post particularly impressive write numbers, exceeding 500 MB/s.
Even the modest 120 GB drive, with just eight die, achieves more than 200 MB/s in our write benchmark. Once the Turbo Write cache is filled, you'll see the 120 GB 840 EVO drop to 130 MB/s or so. And in that product's case, a scant 3 GB of cache fills up a lot more quickly. Even still, the benefit is tangible in real-world measures of performance.
Although Turbo Write complicates our results, it's a solid technology. Moreover, it's now one of several SLC emulation systems in use. SanDisk's nCache is one of the originals, but Toshiba, OCZ, and now Samsung employ mechanisms that are not entirely dissimilar. As you can see, benchmarking and explaining results gets tougher. But their utility does translate into real-world performance improvements. Particularly when you're talking about the smaller drives equipped with fewer dies, the speed-up is meaningful.