MB/s is only a measure that's meaningful for sequential reads where the data can be prefetched. Most enterprise storage is based on applications that read randomly all over the disk (like databases and email servers). The benchmark measurement for this type of application is in the number of operations you can do per second. A single hard drive spindle can do between 80 and 150 IO/s, which would generate the number of IO/s times the size of the IO blocks per second.