Cost-Effectiveness Of Raid Schemes - HP 381513-B21 - Smart Array P800 Controller RAID Technology Brief

Cost-effectiveness of RAID schemes

The cost effectiveness of each RAID solution is a balance between the total cost of the array and its
usable capacity. While the total cost includes all the drives in the array, the usable capacity includes
only the drives that store non-redundant (not parity or mirrored) data. One way to evaluate cost
effectiveness is to compare the cost per gigabyte of usable capacity of various RAID levels. Another
useful way to evaluate cost effectiveness is to compare storage efficiency—the usable capacity
divided by the total of capacity of all the drives.
Note:
RAID 6 is supported on a variety of Smart Array Controllers. The
complete list of controllers and support requirements is available
online at this URL: www.hp.com/products/smartarray.
An important factor to note is that the usable capacity of any RAID array is limited by the size of the
smallest hard drive in the array; the extra capacity on larger drives goes unused. For example, an
array with four drives (40 GB, 60 GB, 60 GB, and 60 GB) would have a usable capacity of 4 x 40
GB, or 160 GB. To maximize storage efficiency, all RAID array drives should have the same
capacity. If drives with different capacities are attached to the same controller, it is possible to create
multiple arrays that contain only drives of the same capacity.
Table 2 lists the storage efficiencies of the various RAID levels. The storage efficiency of RAID 1 and
RAID 1+0 is constant, but the storage efficiency of RAID 5 and RAID 6 varies with the number of
drives. The number of parity drives in RAID 5 and RAID 6 schemes is fixed (one parity drive for RAID
5 and two parity drives for RAID 6), so their storage efficiency increases with the number of drives.
As shown in Table 2, RAID 1 and RAID 1+0 have the lowest storage efficiency at 50 percent;
therefore, they are less cost-effective solutions for large arrays. RAID 5 and RAID 6 have much higher
storage efficiencies, but the level of efficiency depends on the number of drives in the array. For a
given number of drives, RAID 5 will have higher storage efficiency than RAID 6; but this difference
shrinks as the number of drives increases. The storage efficiency of a RAID 5 array varies from
67 percent for three drives to 93 percent for 14 drives (the maximum recommended by HP). The
storage efficiency of RAID 6 varies from 50 percent for four drives to 96 percent for specific storage
systems. The maximum number of physical drives that each HP Smart Array controller can support is
identified on this web page: www.hp.com/products/smartarray
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