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 above 400 MB/s and stay high for long, until around 340 GB of data have been written, at which point the pseudo SLC cache is full and the drive has to write directly to TLC. Note how the SLC cache size matches a third of the total capacity of the SSD exactly—all flash cells get filled in SLC mode first, which consumes three times the storage, but at much faster rates. Once the SLC cache is full, the drive has to shuffle data out of the SLC cache and into TLC, at the same time as storing new incoming data. That's why the rates in this state are much lower, around 80 MB/s, which is slower than many HDDs.