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 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 out at over 5 GB/s, which is very impressive. The drive sustains these write rates until 600 GB have been written. At this point, the SLC cache is full, and the drive will start flushing SLC back to TLC, which has an effect on write rates. Transfer rates beyond this point vary quite a bit. For a hundred GB or so, you'll see 2 GB/s, after which speeds drop to 800 MB/s, with occasional bumps to 2 GB/s.

Filling the whole drive completes at 1240 MB/s, which is a very decent result, better than most TLC drives out there, but still considerably slower than what the WD Black SN850 and Samsung 970/980 Pro offer.

Please note that the SLC cache appears quite large in the chart below because this is a 2 TB drive. On a 1 TB drive, I'd assume the SLC cache to be half that, 315 GB or so.

Any pause in write activity, even for a second, will give the drive time to free up some SLC cache, so full write rates are restored after some idle time even if the drive is partially filled.