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 almost 4 GB/s, which is sustained until 231 GB have been written to the drive. This is a big SLC cache, which is definitely useful to hide the slow write rates of the QLC flash. Once the SLC cache is full, write speeds drop A LOT, down to around 100 MB/s, which is actually slower than most HDDs. Filling the whole capacity completes at 122 MB/s on average, which is one of the lowest results in our test group, lower than all other QLC drives, except for the SATA-based Samsung 870 QVO.