pSLC Cache / 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 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, which is something most consumers will never do.


Write speed starts out at over 1.6 GB/s and is sustained very well thanks to a large SLC cache. With 234 GB, the cache is pretty much as big as possible for a 1 TB QLC drive. 1 TB of QLC flash (4-bits-per-cell) in SLC mode (1-bit-per-cell) equals 250 GB. Such a big SLC cache will be useful for soaking up incoming write bursts. Once the SLC cache is exhausted, the drive has to juggle handling incoming writes and moving data out of SLC into QLC to make up free space for the incoming data. Here, write rates are extremely low, just 100 MB/s is HDD territory and one of the lowest results we've ever seen in this test. Filling the whole drive completes at 139 MB/s, which is a very bad result, too. When write activity stops and the SLC cache has had time to free up some capacity, full write rates are restored within seconds.