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 over 4 GB/s, which is sustained until 638 GB have been written to the drive. This is a big SLC cache, the drive will fill all its capacity in SLC mode first (638 GB x 3 bits TLC = 1914 GB). Once the SLC cache is exhausted, the drive has to move existing data from pseudo-SLC into TLC and handle incoming writes at the same time, which lowers write speeds considerably. In this state, you're getting speeds of 250 to 500 MB/s, which is much lower than 4 GB/s, but still much faster than the 100 MB/s folding speed of competing QLC drives. Filling the whole capacity completed at 442 MB/s on average, which is definitely not "fast," but still several times higher than the direct competitors.