HP 166207-B21 Compaq RAID Advanced Data Guarding: A Cost-effective, Fault-tole - Page 7

TC020604TB TECHNOLOGY BRIEF Compaq RAID Advanced Data Guarding ... RAID ADG IS A COST-EFFECTIVE SOLUTION 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. 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 ADG varies with the number of drives. The number of parity drives in RAID 5 and RAID ADG schemes is fixed (one parity drive for RAID 5 and two parity drives for RAID ADG), so their storage efficiency increases with the number of drives. As shown in Table 2, RAID 1 and 1+0 have the lowest storage efficiency at 50 percent; therefore, they are less cost-effective solutions for large arrays. A comparison of the storage efficiencies of RAID 5 and RAID ADG requires closer scrutiny. For a given number of drives, RAID 5 will have higher storage efficiency than RAID ADG, although 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 Compaq). The storage efficiency of RAID ADG varies from 50 percent for four drives to 96 percent for a maximum of 56 drives. A maximum of 56 drives can be physically connected to a four-channel controller using four external Compaq storage systems. The bottom line is that RAID 1 and 1+0 have very low storage efficiency. RAID 5 and RAID ADG have much higher storage efficiencies, but the level of efficiency depends on the number of drives in the array. Table 2. Summary of RAID Array Storage Efficiency* RAID 1 RAID 1+0 RAID 5 Advanced Data Guarding Usable Capacity (C= capacity of smallest drive; n= number of drives) C*(n/2) C*(n/2) C*(n-1) C*(n-2) Minimum number of drives 2 4 3 4 Maximum recommended N/A N/A 14 N/A number of drives* Storage efficiency from 50% 50% 67% to %93 50% to 96% minimum to maximum recommended no. of drives** *Compaq recommends not exceeding these maximum figures (excluding any allowable online spares) when configuring a drive array, due to the increased likelihood of drive array failure if more hard drives are added. **The value for storage efficiency is calculated assuming all drives in the array have the same capacity with no online spares. 7