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 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 speeds start out at an extremely impressive 2.3 GB/s and stay at that level until two-thirds (!) of the drive have been filled. This is really impressive; usually, NVMe SSDs have only a small SLC cache of around 10-20 GB, for which they will write at full speed, and after a few seconds they drop to lower speeds. Not so with the XPG Spectrix S40G — only after 700 GB have been written, speeds are lowered to around 400 MB/s, which is comparable to other TLC drives on the market. Considering that this drive is 1 TB TLC, there's no way 700 GB can be written in SLC mode (which uses 3x as much storage). Rather it seems that Realtek is using MLC caching on their drive, which is something we've seen from Realtek before. Once write activity stops and the drive is idle, the pSLC cache gets flushed to TLC in the background and full write performance is restored.