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 well over 5 GB/s, which is sustained until 679 GB have been written to the drive. This is a huge SLC cache and actually means the drive fills nearly its whole capacity in SLC mode first—679 GB in SLC are 2037 GB in TLC mode. Once the SLC cache is full, the drive has to flush existing data from SLC to TLC while processing incoming data. It handles this quite well, at over 1.5 GB/s. Filling the whole capacity completes at 2 GB/s on average, which is one of the best results we've ever seen from a TLC SSD.