Compaq 166207-B21 - Smart Array 5302/32 RAID Controller Technology Brief page 7

TC020604TB
T B
ECHNOLOGY RIEF
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R A I D A D G
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I S A
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The cost effectiveness of each RAID solution is a balance between the total cost of the array and
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its usable capacity. While the total cost includes all the drives in the array, the usable capacity
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includes only the drives that store non-redundant (not parity or mirrored) data. One way to
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evaluate cost effectiveness is to compare the cost per gigabyte of usable capacity of various RAID
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levels. Another useful way to evaluate cost effectiveness is to compare storage efficiency—the
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usable capacity divided by the total of capacity of all the drives.
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An important factor to note is that the usable capacity of any RAID array is limited by the size of
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the smallest hard drive in the array; the extra capacity on larger drives goes unused. For example,
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an array with four drives (40 GB, 60 GB, 60 GB, and 60 GB) would have a usable capacity of 4 x
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40 GB, or 160 GB. To maximize storage efficiency, all RAID array drives should have the same
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capacity. If drives with different capacities are attached to the same controller, it is possible to
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create multiple arrays that contain only drives of the same capacity.
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Table 2 lists the storage efficiencies of the various RAID levels. The storage efficiency of RAID 1
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and RAID 1+0 is constant, but the storage efficiency of RAID 5 and RAID ADG varies with the
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number of drives. The number of parity drives in RAID 5 and RAID ADG schemes is fixed (one
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parity drive for RAID 5 and two parity drives for RAID ADG), so their storage efficiency
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increases with the number of drives.
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As shown in Table 2, RAID 1 and 1+0 have the lowest storage efficiency at 50 percent; therefore,
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they are less cost-effective solutions for large arrays. A comparison of the storage efficiencies of
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. . RAID 5 and RAID ADG requires closer scrutiny. For a given number of drives, RAID 5 will have
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higher storage efficiency than RAID ADG, although this difference shrinks as the number of
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. . drives increases. The storage efficiency of a RAID 5 array varies from 67 percent for three drives
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to 93 percent for 14 drives (the maximum recommended by Compaq). The storage efficiency of
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. . RAID ADG varies from 50 percent for four drives to 96 percent for a maximum of 56 drives. A
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maximum of 56 drives can be physically connected to a four-channel controller using four external
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. . Compaq storage systems.
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The bottom line is that RAID 1 and 1+0 have very low storage efficiency. RAID 5 and RAID
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ADG have much higher storage efficiencies, but the level of efficiency depends on the number of
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drives in the array.
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Table 2. Summary of RAID Array Storage Efficiency
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Usable Capacity
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(C= capacity of smallest
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drive; n= number of drives)
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Minimum number of drives
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Maximum recommended
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number of drives*
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Storage efficiency from
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minimum to maximum
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recommended no. of drives**
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*Compaq recommends not exceeding these maximum figures (excluding any allowable online spares) when configuring
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a drive array, due to the increased likelihood of drive array failure if more hard drives are added.
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**The value for storage efficiency is calculated assuming all drives in the array have the same capacity with no online
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spares.
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7
Compaq RAID Advanced Data Guarding
C - E
O S T F F E C T I V E
RAID 1 RAID 1+0
C * (n/2) C * (n/2)
2 4
N/A N/A
50% 50%
S
O L U T I O N
*
RAID 5 Advanced Data
Guarding
C * (n-1)
3
14
67% to %93 50% to 96%
C * (n-2)
4
N/A