pSLC Cache / Write Intensive Usage

When copying games from your Steam Library or other very large files (>10 GB), you might have noticed that write speeds on your SSD start out at full speed and then drop considerably. The underlying reason is that modern drives have 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, which is something most consumers will never do.


Write speeds start at the near SATA maximum of 450 MB/s, but are only sustained for a short time, until 20 GB have been written, after which speeds drop considerably, to around 150 MB/s, which is in the territory of classic HDDs. Filling the whole 1 TB capacity completes at 197 MB/s—better than QLC, but well behind most TLC-based drives. When write activity stops and the SLC cache has had time to free up some capacity, full write rates are restored within seconds.