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 as well, which can result in a significant loss of write speed. Newer TLC drives operate part of their capacity in SLC mode for increased performance. This test can reveal the size of that 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.


WD quotes "up to 2800 MB/s" write speeds. The reality is that those speeds will only be achieved for 2-3 seconds (!), after which write speeds will drop down a constant 800 MB/s, which is a good result, but of course only a fraction of the rated "up to" value. The pseudo-SLC cache is really small at just 6 GB and gets exhausted very quickly, especially given the high burst write speeds. Competing SSDs offer much bigger caches, which help soak up larger write bursts more easily. An averaged write speed of 879 MB/s is good, better than most TLC-based NVMe SSDs on the market. Once the write activity stops and the drive is idle, the pSLC cache gets flushed to TLC in the background and full write performance is restored.