pSLC Cache / Write Intensive Usage

When copying games from your Steam Library, or other very large files (>10 GB), you may notice that write speeds on your SSD start at full speed, before dropping considerably. The underlying reason is that modern drives have small, fast caches, that soak up write bursts to improve performance. In the fairly uncommon scenario of writing data that's too big to fit into these caches, the drive will have to write data directly to flash, and it will probably juggle some out of its write cache at the same time, which can result in a significant loss of write speed. Newer TLC drives use part of their capacity in SLC mode for increased performance. This test can reveal the size of that pseudo-SLC cache.

Testing on this page looks at exactly that scenario. We write a sequential stream of 1 MB blocks to the drive in a single thread, like a typical file-copy operation would do, and measure write speeds twice a second. The drive is fully erased before testing to ensure any caches are emptied. Please note that this test writes a lot of data in a very short time, something most consumers will never do.


On a freshly erased drive, write speeds start at a surprisingly low 1.0 GB/s, just like on the Hynix P41. I've tested this several times and it happened with multiple test programs, so it's not a measurement error. It seems that the first 16 GB written to the drive are limited to 1 GB/s. I have no idea why, but it's probably a firmware bug. I've notified Solidigm and will update this section if I get feedback from them. However, it is a non-issue as it happens only on the first 16 GB written to a fresh drive—i.e., during the Windows installation or when filling the drive for the first time. Reading back the data stored in those first 16 GB always runs at full speed, so it's more of a curiosity than an actual issue.

Once you're past 16 GB, speeds jump to almost 5 GB/s—very impressive. These speeds are sustained until 289 GB have been written, which is a large SLC cache size. Once the SLC cache is full, the SSD will write directly to TLC flash, which makes write rates drop to slightly under 2 GB/s. Once 1.5 TB have been written, there's a surprising jump to full write speeds, but speeds return to the expected 1.8 GB/s after a short while.

Filling the whole capacity completed at 1971 MB/s, which is a fantastic result, better than most SSDs we've tested; slightly faster than the Samsung 980 Pro and a bit faster than the WD Black SN850. That having been said, some Phison E18+Micron B47R drives are faster. When write activity stops and the SLC cache has had time to free up some capacity, full write rates are restored within seconds.


The 1 TB variant shows exactly the same behavior, but comes with a smaller SLC cache of 198 GB. Filling the whole capacity of the 1 TB drive completed at 1768 MB/s