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, 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.
Just like all other modern SSDs, the Samsung 870 QVO has pseudo-SLC caching, which operates part of the total capacity in SLC mode, which is much faster than writing to QLC directly. When writing to an empty cache, we see excellent transfer rates of more than 500 GB/s—right at the limit of the SATA interface. Once you've written 42 GB, transfer rates fall off a cliff to below 100 MB/s—that is slower than most HDDs. It seems writing to QLC directly, possibly while trying to flush the SLC cache in the background, is a huge issue for the 870 QVO. At those rates it is the slowest SSD we have ever put through this test. The SLC cache is also relatively small, other QLC drives like the Crucial P1 and Sabrent Rocket Q have caches several times bigger.
Once the write activity stops and you give the 870 QVO enough time to flush the cache from SLC to QLC, write performance is restored.