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 speeds start at the near SATA maximum of 450 MB/s, and are sustained for a surprisingly long time, until 660 GB have been written to the drive. This means that the Vulcan Z will fill its whole capacity in SLC mode first. When writing to TLC in SLC mode, three times the storage is consumed for each byte, so 660 GB x 3 =1980 GB. Once the drive is full and more data is incoming, speeds will drop a lot, because the drive has to move data out of SLC and write it back into TLC, which is a much slower process. In this state you're getting around 90 MB/s, which is slower than many classic HDDs. Filling the whole 1 TB capacity completes at 145 MB/s—better than QLC, but well behind most TLC-based drives. When write activity stops and the SLC cache has had time to free up some capacity, full write rates are restored within seconds.