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 speeds reach almost 5 GB/s. These speeds are sustained until 615 GB have been written, which means the drive will fill 45% of its capacity in SLC mode first. Once the SLC cache is full, write speeds drop to around 2 GB/s and then to just 600 MB/s once 2600 GB have been written. Filling the whole capacity completes at 1533 MB/s on average, a fantastic result for a value-oriented DRAM-less drive—the best in our test group, and very similar to what high-end drives with DRAM achieve.