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 7 GB/s, which is very impressive for single-threaded write speeds. These speeds are sustained until 660 GB have been written, which equals to 1980 GB in TLC mode—the drive is almost completely full now. At this point the controller has to copy data out of SLC cache into TLC, while processing incoming writes at the same time. Here the speeds drop considerably, to around 1 GB/s, which is still pretty good, but just 10% of what the "10 GB/s PCIe Gen 5" promise suggests. Filling the whole capacity completes at 1467 MB/s on average, a result that's comparable to the best Gen 4 drives in our test group, but not significantly better.