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 5 GB/s, which is very good for single-threaded write speeds. These speeds are sustained until 54 GB have been written, after that speeds drop slightly to 3.5 GB/s. Once 460 GB have been written, the SLC cache is full and write performance drops a lot—down to around 130 MB/s, which is HDD territory. Pausing the stream of incoming data will of course restore full write speeds, because the SLC cache has time to flush itself to QLC. Filling the whole capacity completed at 168 MB/s on average, which is one of the lowest results we've seen in this testing, only the SATA-based Samsung 870 QVO is slower here.