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 238 GB have been written, which means the drive will fill around 20% of its capacity in SLC mode first, which is a relatively small cache (when taking 4 TB into account), but still big enough for nearly all workloads. Once the SLC cache is full, write speeds drop to 4 GB/s, which is sustained until around 25% of the drive is full, then speeds drop to 2 GB/s and after 2.5 TB to 800 MB/s. Pausing the stream of incoming data will of course restore full write speeds, because the SLC cache has time to flush itself to TLC. Filling the whole capacity completed at 1.3 GB/s on average, a good result, but considerably lower than some other recent releases.