pSLC Cache / Write Intensive Usage

When copying games from your Steam Library, or other very large files (>10 GB), you may notice that write speeds on your SSD start at full speed, before dropping considerably. The underlying reason is that modern drives have small, fast 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, something most consumers will never do.


Write speed starts out at above 5 GB/s, which is sustained for almost 1.3 TB, which means the drive will fill all its capacity in SLC mode. Writing a bit in SLC occupies three bits, so 3x 1.3 TB = 3.9 TB. Once the the SLC cache is full, the drive will start flushing SLC back to TLC, which affects write rates. Here you're getting 1-2 GB/s, depending on how full the drive is, still very good. Filling the whole 4 TB capacity completed at 1.85 GB/s average, an excellent result.