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 speed starts out at 3 GB/s, which is sustained until 63 GB have been written to the drive. This is a relatively small SLC cache by today's standards, where we're often seeing 100 GB or more. Once 63 GB have been written, the SLC cache is full, and the drive will start flushing SLC back to TLC; here, we see performance fall off a cliff in a completely unexpected way. Strangely, once the cache is full, write speeds drop to 150 MB/s, only to recover a bit later, to 1200 MB/s. I've never seen such a pattern on any other SSD before, which suggests it's the way Phison is doing things with their new E15T controller, or it could be some sort of firmware bug. We'll encounter that same pattern again in the thermal test on the next page.

With 950 MB/s to fill the whole drive, the M390 achieved a "middling" result that's considerably better than some value M.2 NVMe SSDs which sit at around 500 MB/s, but also lower than more premium models, which reach well above 1 GB/s.

When write activity stops and the SLC cache has had time to free up some capacity, full write rates are restored even if the drive is partially filled.