pSLC Cache / Write Intensive Usage

When copying games from your Steam Library or other very large files (>10 GB), you might have noticed that write speeds on your SSD start out at full speed and then drop considerably. The underlying reason is that modern drives have 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, which is something most consumers will never do.


Write speeds starts out at over 4.5 GB/s—very impressive. These speeds are sustained until 280 GB have been written, which is a large SLC cache size. Once the SLC cache is full, the SSD will write directly to TLC flash, which makes write rates drop to roughly 2 GB/s until almost 500 GB have been written. Beyond that point, the drive starts moving data out of SLC into TLC to free space for incoming data, which results in quite low speeds of 500 MB/s until the full capacity is used up.

Filling the whole 1 TB capacity completed at 830 MB/s, which is a good result considering this is a DRAM-less drive, but high-end DRAM-based PCIe 4.0 SSDs run much faster when the SLC cache is full, reaching more than twice the speed. When write activity stops and the SLC cache has had time to free up some capacity, full write rates are restored within seconds.