14 OverviewDisk mirroring provides 100 percent redundancy, but is expensive because each physical disk in thesystem must be duplicated. Figure 1-2 shows an example of disk mirroring.NOTE: Mirrored physical disks improve read performance by read load balance.Figure 1-2. Example of Disk Mirroring (RAID 1)ParityParity creates a set of redundant data from two or more parent data sets. You can use the redundant datato rebuild one of the parent data sets. Parity data does not fully duplicate the parent data sets but thatdata can be used to reconstruct the data if lost. In RAID, this method is applied to entire physical disksor stripes across all the physical disks in a physical disk group.The parity data is distributed across all the physical disks in the system. If a single physical disk fails, itcan be rebuilt from the parity and the data on the remaining physical disks. RAID level 5 combinesdistributed parity with disk striping, as shown in Figure 1-3. Parity provides redundancy for one physicaldisk failure without duplicating the contents of entire physical disks. However, parity generation can slowthe write process.Figure 1-3. Example of Distributed Parity (RAID 5)NOTE: Parity is distributed across multiple physical disks in the disk group.Stripe element 1Stripe element 2Stripe element 3Stripe element 1 DuplicatedStripe element 2 DuplicatedStripe element 3 DuplicatedStripe element 4 Stripe element 4 DuplicatedSegment 1Segment 7Segment 2Segment 8Segment 3Segment 9Segment 4Segment 10Segment 5Parity (6–10)Parity (11–15)Parity (1–5)Segment 6Note: Parity is distributed across all drives in the array.Segment 12Segment 15 Segment 11Segment 14Segment 13Segment 19Segment 25Segment 20Segment 23Segment 18Segment 21Segment 16Segment 22Segment 17Parity (21–25)Parity (26–30)Parity (16–20)Segment 24Segment 30Segment 27 Segment 29Segment 26 Segment 28Dell_PERC5_UG.book Page 14 Tuesday, February 13, 2007 6:02 PM
