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Number of Drives

The number of hard drives in the system has an important impact on both capacity and performance. Many RAID enthusiasts believe that the more drives you put in the array, the better off you are--and this usually is mostly true. At the same time, there are also disadvantages to using more drives as opposed to fewer.

The number of drives in the array affects the following aspects of the array directly:

• Capacity and Storage Efficiency: The size of the array you end up with depends on the RAID level implemented and the number of drives in the array (as well as the size of each drive, of course). Storage efficiency, meaning the percentage of the total amount of space on all the drives that contains user data, is also a function of the number of drives in the array for all RAID levels that use striping with parity (the number of parity drives is fixed so the efficiency goes up as you add more drives.) See this discussion of capacity and storage efficiency for more.
• Performance: For RAID arrays that use striping, the stripe width (not stripe size) is equal to the number of disks in the array. Adding more drives improves both random and sequential performance; in the case of sequential performance, theoretical throughput is roughly proportional to the number of drives in the array (with random performance it is more complicated).
• Cost: More drives cost more money, even if you don't increase the capacity of the array. Ten 20 GB drives is 200 GB, but it will cost more than five 40 GB drives do. Then, above the cost of the drives, you have to consider support costs: more drives take more space, need more power, and often require more cooling. Going to too many drives may require the purchase of an extra enclosure, and this incremental cost alone can dwarf the other hardware costs for a RAID array in a smaller workstation or server. Also, see here for issues related to power and cabling.
• Reliability: More hardware means more to potentially fail; the reliability of the overall system will go down when more drives are involved. See here for more.

The #1 question that people ask about the number of drives to use in an array goes something like this: "Should I use three 36 GB drives, or six 18 GB drives?" The answer, of course, is "it depends". For people who really want to maximize performance--which is most people--the answer is six 18 GB drives. However, if you do this, you must be prepared for the extra costs. As mentioned above, sometimes the deciding factor is how many drives the system can hold; it might be ideal to make a striped array out of twelve 9 GB drives instead of either of the options above, but few systems can handle twelve drives without adding an expensive external enclosure, which totally changes the cost picture of the entire system. The capabilities of the controller can also enter the picture, as they will often have a limit on the number of drives they will support.

Next: Drive Size

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