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 around 8.5 GB/s. This rate is sustained until 720 GB have been written, which means the drive will fill around 53% of its capacity in SLC mode first, where each bit consumes three actual bits of storage. That's the second-biggest SLC cache in our whole test group, and far more than you're ever going to need in a normal day of use. Once the SLC cache is exhausted, write speeds still reach around 3 GB/s, which is an excellent result for writing straight into TLC. Filling the whole 4 TB capacity completes at 3.1 GB/s on average—an extremely impressive result, only the Corsair MP700 Pro XT does better, and it needs a lot more power to get there.

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.