The two larger SM951s yield the best throughput of the drives on our chart, but the higher-capacity XP941s complete the tasks in less time. The Lenovo-specific SM951 lands just behind last year's OEM superstar from Samsung. Meanwhile, the smaller SM951s take longer to finish the tasks under light workload conditions coming out of heavy workloads.
Lower-capacity Samsung SSDs are hit and miss when it comes to cleaning up the NAND after heavy workloads. Many of the company's products displayed this behavior. What we're seeing is the clean-up process taking longer. New reads and writes happening at the same time as background operations increases latency. I noticed this with the 830- and 840-series drives. The 850s with 3D V-NAND were more resilient, even after especially taxing workloads. The SM951 uses 1xnm 2D planar MLC NAND that evolved from what we found in the 840-series, though.
The light workload set assumes that five minutes between tests is long enough for the drive to tidy up dirty cells. So either it takes longer to clean house, or the house gets dirty faster with the large 128Gb dies. Both the 128GB and 256GB SM951s use just two NAND packages and the 512GB model uses four. This can limit I/O, even with quad-plane packages.
Here we see the 128GB SM951 in a dirty state after a reasonable number of sequential and random writes. At this point in the test process, the drive wasn't being pushed particularly hard. Despite that, 128KB sequential write performance is already down to very low levels. Over time, the internal wear-leveling algorithms will clean the flash and write performance should recover where cells are free.