When you buy a disk, you are really buying two things: storage capacity and throughput capacity. For example, if you need to buy 72 gigabytes (GB) of capacity, you can purchase a single 72GB unit or four 18GB units. The storage capacities of these disks are exactly the same, but the four-drive configuration potentially has four times greater throughput capacity. Each drive is capable of doing approximately 100 input/output (I/O) operations per second regardless of its size. Therefore the four-drive system has the potential to perform 400 I/O operations per second.
For several reasons, it is generally less expensive to purchase fewer larger disks to get to your desired capacity:
The 72GB drive is only marginally more expensive than a single 18GB drive, so buying four 18GB drives to obtain the same capacity as a 72GB drive will substantially increase your cost.
You must have the storage space to hold the additional drives; more storage space can lead to additional cost.
You might need more controllers to efficiently run the additional disks, also adding to the cost.
However, if performance is more important than cost saving, you should purchase a greater number of physical disk drives to give you the greatest throughput potential. The additional cost of the multi-disk solution can be offset by increases in performance (user efficiency, programmer time, customer loyalty) over time.
If you are using a database only as archival storage, and performance is not critical, fewer large disks are recommended to keep costs lower.
The 72GB drive is only marginally more expensive than a single 18GB drive, so buying four 18GB drives to obtain the same capacity as a 72GB drive will substantially increase your cost.