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 around 2.8 GB/s, which is surprisingly low for a "PCIe 4.0 SSD", and in the range of what we're usually seeing on Gen 3 SSDs. The underlying reason is that there's only four flash channels, so writing is slower. Once 88 GB have been written, the pseudo-SLC cache is full, and the drive has to write to flash directly. 88 GB for a 1 TB SSD in 2022 is relatively small, but still ok. Writes now run at a very respectable 600 MB/s—considerably faster than what SATA drives offer. Only once 800 GB have been written to the drive, performance starts jumping between 600 and 80 MB/s, averaging to around 400 MB/s, until the drive is full. This is a HUGE improvement compared to the Kingston NV1, which dropped to approximately 100 MB/s once the SLC cache was full.

Filling the whole 1 TB capacity completed at 620 MB/s average, a good result, comparable to other DRAM-less SSDs like the Samsung 980, WD Black SN770 and others.