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/QLC drives use part of their capacity in SLC mode for increased performance. This test can reveal the size of that pseudo-SLC cache.

The testing methodology used on this page simulates such a 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. Please note that some other publications test this at higher queue depth, which will yield bigger numbers, that are less relevant to real-world scenarios. The drive is fully erased before testing, to ensure any caches are emptied. This test generates a heavy write load with lots of data, something that most people don't do regularly—typical consumer workloads involve shorter bursts of write activity.


Write speed starts out at 8 GB/s, which is very impressive for single-threaded write speeds. These speeds are sustained until 368 GB have been written, which means the drive will fill 54% of its capacity in SLC mode first, where each bit consumes three actual bits of storage. Once the SLC cache is full, write speeds drop to around 4 GB/s—still plenty fast. Once you go beyond 1.35 TB written, speeds drop again, to 2 GB/s, which is still faster than most drives on the market with full cache. Filling the whole capacity completes at 3.3 GB/s on average, which is one of the best results that we've ever seen in this test.

As soon as the incoming stream of data stops, even for a few seconds, write speeds will be restored to full, because the SLC caching process has had some time to free up capacity in the background.