There was a time when the only media you could get in a shared storage array was the humble hard disk drive. Although early systems used DRAM (system memory) as a cache, overall performance was always limited by the capability of the drives themselves, whether that was from the perspective of latency or throughput. Over time, arrays became more diversified with the implementation of tiered storage. Tiering was done to save cost, and it placed drives (and eventually flash) of different performance characteristics into the same array, moving data around using intelligent placement algorithms to determine the optimum location for each piece of data. The aim was to place data on the most appropriate tier to match the I/O requirements of the application, without spending money on expensive storage when it wasn’t needed.