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 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.


Sequential write speeds start out very high at 1.6 GB/s while the pseudo-SLC cache gets filled up. Once it is exhausted (after around 140 GB of writes), the drive has to write to its QLC flash directly, dropping speeds to only 150 MB/s, which is really slow. However, you have to consider that it's highly unlikely any consumer will write that much data in one go. What's great is that the pSLC buffer is big—on many TLC drives, it's in the order of 4 GB to 16 GB. Crucial opted for a much larger one capable of soaking up all but the biggest write bursts. Once the write activity stops and the drive is idle, the pSLC cache gets flushed to QLC in the background, and full write performance is restored.