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.


Write speed starts out at around 4.5 GB/s, which is very good for single-threaded write speeds. These speeds are sustained until 1215 GB have been written, which means the drive will fill almost its whole capacity in SLC mode first (1215 GB x 3 = 3645 GB = 95.5%). Once the SLC cache is full, write speeds drop to around 700 MB/s. Surprisingly, once 2.4 TB have been written, speeds recover and go up to over 3 GB/s again. Filling the whole capacity completes at 1420 MB/s on average, a very decent result for the drive's positioning and better than many value-drive competitors that don't even get close to 1 GB/s in this same test.