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 probably juggle some out of write cache at the same time, too, which can result in a significant loss of write speed. Newer TLC drives operate 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 empty. Please be aware that this test writes a lot of data in a very short time, which is something most consumers will never do.


We see the drive run at full write speed until 16 GB have been written before performance drops significantly. The underlying reason is that 16 GB seems to be the size of the DRAM cache + SLC cache. Once that is exhausted, data has to be written to TLC flash directly, which is a much slower operation. In that state, performance is quite unpredictable and ends up at only 173 MB/s when averaged over time - a speed comparable to traditional HDDs.