150 IBM System Storage N series Hardware Guide10.2.1 Single-parity RAID using larger disksThe various options to extend the ability of single-parity RAID to protect data as diskscontinue to get larger are unattractive. The first option is to continue to buy and implementstorage that uses the smallest disk sizes possible so that reconstruction completes quicker.However, this approach is impractical. Capacity density is critical in space-constrained datacenters, and smaller disks result in less capacity per square foot. Also, storage vendors areforced to offer products that are based on what disk manufacturers are supplying, and smallerdisks are not readily available, if at all.The second way to protect data on larger disks with single-parity RAID is slightly morepractical, but still not effective for various reasons. Keeping the size of arrays or volumessmall, the time to reconstruct is reduced. However, an array or volume that is built with moredisks takes longer to reconstruct data from one failed disk than one built with fewer disks.Smaller arrays and volumes have the following costs that cannot be overcome: More disks are lost to parity, which reduces usable capacity and increases the total cost ofownership (TCO). Performance is slower with smaller arrays, aggregates, and volumes, which affectsbusinesses and users.The most reliable protection that is offered by single-parity RAID is RAID 1, or mirroring. InRAID 1, the mirroring process replicates an exact copy of all data on an array, aggregate, orvolume to a second array or volume. Although RAID 1 mirroring affords maximum faulttolerance from disk failure, the cost of the implementation is severe. RAID 1 requires twice thedisk capacity to store the same amount of data.The use of smaller arrays and volumes to improve fault tolerance increases the total cost ofownership of storage because of less usable capacity per dollar spent. RAID 1 mirror with itsrequirement for double the amount of capacity is the most expensive type of storage solutionwith the highest total cost of ownership, as shown in Figure 10-3.Figure 10-3 RAID 1 mirror10.2.2 Advantages of RAID-DP data protectionBecause the current landscape with larger disk drives affect data protection, customers andanalysts need a way to affordably improve RAID reliability from storage vendors. To meet thisdemand, a new type of RAID protection called RAID Double Parity (RAID-DP) wasdeveloped, as shown in Figure 10-4 on page 151.
