Write Intensive UsageWhen 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 as well, 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 emptied. Please note that this test writes a lot of data in a very short time, which is something most consumers will never do.
As long as the drive is writing to its SLC buffer, speeds are very good, reaching the 500 MB/s limit of the SATA interface. Once the buffer is exhausted though, speeds drop like a rock, down to around 150 MB/s. They do recover after a few seconds (when the SLC buffer has been flushed), but drop again quickly, as long as the write load is running.
Compared to other drives, the SLC cache is rather small with 4 GB.