IBM Power Systems 775 Manual page 94

Figure 1-58 Conventional RAID versus declustered RAID layouts
Figure 1-58 shows an example of how GPFS Native RAID improves client performance
during rebuild operations by using the throughput of all disks in the declustered array. This is
illustrated by comparing a conventional RAID of three arrays versus a declustered array, both
using seven disks. A conventional 1-fault-tolerant 1 + 1 replicated RAID array is shown with
three arrays of two disks each (data and replica strips) and a spare disk for rebuilding. To
decluster this array, the disks are divided into seven tracks, two strips per array. The strips
from each group are then spread across all seven disk positions, for a total of 21 virtual
tracks. The strips of each disk position for every track are then arbitrarily allocated onto the
disks of the declustered array (in this case, by vertically sliding down and compacting the
strips from above). The spare strips are uniformly inserted, one per disk.
As illustrated in Figure 1-59 on page 81, a declustered array significantly shortens the time
that is required to recover from a disk failure, which lowers the rebuild overhead for client
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IBM Power Systems 775 for AIX and Linux HPC Solution