As we know, Samsung is in a unique position as an SSD manufacturer. The fact that it fabricates its own NAND means the company can cherry-pick the very best flash for those 840 Pros. It's also pushing the envelope with regard to lithography in a way that very few competitors can match. Consequently, Samsung's engineers also know where those solid-state drives come up short, giving them a problem to work on solving.
When you look at the spec sheet for pretty much any SSD family, you notice that the maximum performance levels are typically available from the highest-capacity models. At lower capacities, those throughput numbers trail off pretty fast. In fact, many drives under 100 GB bashfully offer sequential write rates in the low 100 MB/s range. Why? Usually, it comes down to the number of dies per memory package, and the controller's ability to utilize them all in parallel. As the density of flash increases, you need fewer dies to hit certain capacity points, and that can detract from performance. SSD vendors can use lower-density NAND for as long as it's still available, but that also drives prices up, which nobody likes.
This conundrum is exacerbated by triple-level cell flash. Capable of storing three bits for every memory cell, TLC NAND further increases density and reduces cost compared to MLC flash with two bits per cell. The price you pay is higher latency. TLC NAND takes longer to stabilize, which affects its performance.
To combat those compromises, Samsung introduced us to a couple of new technologies. The first is called Turbo Write. Think of this as an additional tier of storage space between the system RAM and TLC NAND. In effect, Samsung takes a small amount of the triple-level cell flash and treats it like SLC, or single-level cell memory. This reduces latency significantly, augmenting performance so long as the data you're working with is in that space.
Turbo Write increases performance across all capacity points, though the 120 GB configuration enjoys the largest boost. Claimed to improve sequential throughput by as much as 3x, this feature is key to extracting respectable benchmark numbers from a low-cost, lower-capacity SSD.
Samsung based the amount of NAND dedicated to Turbo Write on typical desktop workloads, it said. Even in the entry-level space, the 120 GB 840 EVO can maintain those impressive numbers across 3 GB of writes. That's more than enough for most folks. If you do routinely write more than that at a time, you might want to consider a true enthusiast-oriented SSD like the 840 Pro or one of its competitors.