Write Intensive UsageWhen 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 around 1.7 GB/s, which is quite impressive, especially for a QLC drive, and stay at that level until 250 GB have been written, which is the SLC cache size of the Rocket Q. This is a very large cache size; most other drives only have a fraction of that. Since the Rocket Q is QLC, these 250 GB mean the drive will use SLC mode until it is completely full. QLC is 4 bits per cell, which turns into only 1 bit per cell in SLC mode—a quarter. Like with all other QLC SSDs write speeds drops drastically once the SLC cache has been exhausted, down to around 150 MB/s. As soon as write activity stops and the drive is idle, the pSLC cache gets flushed to QLC in the background and full write performance is restored.