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 3 GB/s, which is super impressive, and stay high until around 47 GB of data have been written—a relatively small pseudo-SLC cache. Looks like Silicon Power was quite conservative with their SLC cache size. While 47 GB is sufficient to soak up many write bursts of activity, competing drives have much bigger caches that are up to 5x bigger, or more. Of course, it is highly unlikely you'll feed the drive such huge write bursts, but I still feel like the SLC cache could be bigger, in the 100 GB range maybe.

Filling the whole drive completes at 1139 MB/s, which is a very good result—better than most other drives, most of which have a larger SLC cache. I guess it's a tradeoff: Would you prefer the ability to quickly handle larger write bursts or better sustained write speed for the drive's whole capacity?

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.