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 speed starts out at well over 5 GB/s, which is sustained until 200 GB have been written to the drive. This means the drive will fill a third of its capacity in SLC mode. This SLC cache size is "good," certainly sufficient for most workloads, but some competing 2 TB drives fill all their capacity in SLC mode first, which lets them soak up even bigger bursts of write activity. Filling the whole 2 TB capacity completed at 1.2 GB/s, which is a below-average result for a PCIe 4.0 SSD. Competing high-end drives reach almost 2 GB/s or above. When write activity stops and the SLC cache has had time to free up some capacity, full write rates are restored even if the drive is partially filled.