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 is a reasonably big SLC cache for a modern SSD, but some drives go higher. As an example, the Kingston KC3000 based on the same Phison E18 controller and Micron 176-layer NAND uses 643 GB SLC cache. While this gives the drive the ability to soak up bigger write bursts, it affects the overall write rate once the SLC cache is full. If you take a look at our "Fill Whole Disk" chart below, you'll see that the KC3000 achieves 2.0 GB/s, whereas the A400 Pro in this review achieves 2.3 GB/s—a significant difference. This is clear evidence that a bigger SLC cache is not automatically better, but a tradeoff. With around 2.5 GB/s once the SLC cache is full, the A440 Pro reaches very impressive speeds in this state, too. 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.