Write Intensive Usage

When copying games from your Steam Library, or other very large files (>10 GB), you might have noticed that write speeds start out at full speed and then drop considerably on your SSD. 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 that write cache, the drive will have to write data directly to flash, and probably juggle some out of write cache at the same time, too, which can result in a significant loss of write speed, especially on TLC-based drives, which implements this cache by using part of the drive's capacity in SLC mode.

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 empty.


As you can see, the drive starts out at nearly 500 MB/s, but then drops to slightly above 400 MB/s. This drop is much smaller than on most other TLC drives, where we have observed speed drops to 200 MB/s or less. What's also worth mentioning is that the SLC cache is written back to in the background since we can see another high-speed burst at around 750 GB written.


I zoomed into the first section of the graph. Here, we see that the drop happens right after 64 GB have been written, which suggests that the capacity of the SLC write cache is 64 GB.

To provide some extra context, I ran the same test on the Crucial MX300 750 GB and Samsung 850 Evo 256 GB.