Random data reads scale well as the queue depth increases. Incidentally, this is affected by the cache. If you read our SSD reviews, you already understand how caching exerts its influence on the performance of storage hardware. Even without Storage Spaces' tiering, WSS makes use of available DRAM to increase I/O performance.
Different products use cache differently. Here, we see Thecus' N5810Pro caching random data writes. The heavy waves demonstrate the appliances strong performance when data is in the cache, and lower performance after a cache flush when information is not ready to be read back. A cache miss, demonstrated by the N5810Pro at a queue depth of 128 in the 8KB chart, can cause very slow performance.
The W5000 delivers smooth random write speeds, though its IOPS are troubling. A result of less than 100 IOPS with both 4KB and 8KB blocks is not good.