Sequential Steady State
The two 256GB SM951s deliver nearly identical steady state sequential performance. The same is true for random 4KB steady state. This test stresses architecture and programing more than the physical flash or controller.
The area we want to really focus on is the performance increase over Samsung's 850 Pro 256GB. We often see reader comments that downplay the delta between SATA and PCIe. It's true that, in some applications, one won't be drastically better than the other. It all comes down to the workloads you subject your storage to.
After a reasonable number of sequential and random writes, we tested the SM951-NVMe with HD Tach to watch sequential performance drop off and background activity kick in. Controller makers use proprietary code to handle those tasks. Sometimes the code changes from firmware updates. But most of the time, alterations are implemented with new processor launches.
Samsung's client SSDs are not very aggressive when it comes to TRIM and cleaning dirty cells. Under heavy real-world workloads, we often see performance drop off. All client SSDs transition from fresh-out-of-box to steady state eventually. But the amount of work it takes to get there and the amount of time it takes for the drive to recover is what we're looking for. Samsung's SSDs in particular tend to fall off faster and take longer to recover than other products.
That isn't always a bad thing, though. It means Samsung SSDs wear their flash less because of lower write amplification. In order to clean cells for fresh writes, the entire page of flash needs to be read and the data rewritten without old data that was deleted. Some SSDs are very aggressive with this process, causing the flash to fail faster. Also, the read, erase and write cycle uses power, so aggressive background activity means higher consumption over time. This affects notebook battery life a great deal.