Disk Access Density.

The capacity of disk drives has increased over 6,000 times since 1964 while the raw device performance (seek, latency, and transfer rate) has only increased eight times during the same period. Continual increases in capacity without corresponding performance improvements at the drive level create an imbalance as defined by the ratio called access density. Access density is the ratio of performance measured in I/Os per second to the capacity of the drive, usually measured in gigabytes. If capacity doubled and performance doubled, the access density would remain unchanged. Scaling disks involves more than increasing capacity.

In reality, the access density has steadily declined as the capacity has increased substantially while performance increases have been modest by comparison. The outlook remains much the same for the future. Drive performance will improve somewhat as 10,000+rpm drives emerge and access times are dropping below ten milliseconds in some cases. Larger caches and actuator-level buffers help improve overall subsystem performance and multi-path I/O porting builds aggregate throughput. Even solid-state disks have regained popularity in the open systems (non-OS/390) marketplace.

Continued capacity increases at the device level are now a significant factor in storage subsystem performance and must be managed. longer range discussions include feasibility tests for dual-actuator disks. In general, very high capacity disks are not well suited for I/O intensive applications such as OLTP and careful data placement on these devices pays large dividends. Access density will become even a more important metric, as future disks will significantly drive capacity upward.

                            Disk Access Density
                 Average Device                   Capacity Access Density
Year   Drive    Service Time (ms) I/Os per second   (GB)   (IOs/sec./GB)
1956   RAMAC         1015.0             .98         .005       196.0
1964    2314          112.5            8.90         .029       304.0
1975    3350          36.7             27.25        .317        85.9
1987   3380K          24.6             40.60        1.89        21.5
1996   3390-3         23.2             43.10        8.52        5.00
1998 Cheetah 18       18.0             55.5        18.20        3.05
1996  Elite 23        19.0             52.6        23.40        2.24
1997  Elite 47        19.0             52.6        47.06        1.12
2000   (est.)         12.0             83.3        80.00        1.01