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 well over 4.5 GB/s, which is sustained until 325 GB have been written to the drive. For a 1 TB SSD, this is a very big SLC cache as it spans pretty much the whole capacity of the drive. 325 GB of data written to TLC flash in SLC mode (which takes up 3x as much space) means 975 GB of TLC capacity are used. Once the SLC cache is full, write speeds drop a lot though, down to around 500 MB/s. Filling the whole 1 TB capacity completes at 630 MB/s on average, which is a decent result for a value-orientated SSD, but far from the 2 GB/s we're seeing on the best high-end Gen 4 drives. When write activity stops and the SLC cache has had time to free up some capacity, full write rates are restored even if the drive is partially filled.