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 excellent 1.6 GB/s and stay very high and constant until around 700 GB have been written to the drive. Then, performance drops off a cliff, down to around 200 MB/s.

This behavior is completely different to what we've seen in previous SSD reviews. It looks like the Realtek controller doesn't use SLC caching at all. A 1 TB SSD with all its cells written in SLC mode has a capacity of 333 GB, but why can the SX6000 Pro perform at full speed for almost twice that capacity then? The answer seems to be that instead of SLC, the controller uses MLC, which stores two bits per cell (= 666 GB capacity in MLC mode). Further, the data suggest that the drive will fill up all its available capacity with MLC caching—once exhausted, it has to copy existing data from MLC into TLC to free up space to reach its advertised capacity, all while handling the incoming stream of data, which explains the poor write speeds beyond that point.

With an average write speed of 500 MB/s, it still does much better than QLC-based drives and is comparable to earlier TLC-based NVMe disks.