Crystaldiskmark !full!

This visualizes the dynamic. Most modern SSDs use a portion of their capacity as a high-speed cache (simulating faster SLC memory) to absorb burst writes. Once that cache is full, the drive must write directly to the slower native NAND. CrystalDiskMark is frequently used to test exactly how large that cache is and what the "sustained" write speed looks like after the cache is exhausted—a critical factor for video editors.