Dell PowerEdge RAID Controller 5i - Manual (Page 16)

14
Overview
Disk mirroring provides 100 percent redundancy, but is expensive because each physical disk in the
system 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)
Parity
Parity creates a set of redundant data from two or more parent data sets. You can use the redundant data
to rebuild one of the parent data sets. Parity data does not fully duplicate the parent data sets but that
data can be used to reconstruct the data if lost. In RAID, this method is applied to entire physical disks
or 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, it
can be rebuilt from the parity and the data on the remaining physical disks. RAID level 5 combines
distributed parity with disk striping, as shown in Figure 1-3. Parity provides redundancy for one physical
disk failure without duplicating the contents of entire physical disks. However, parity generation can slow
the 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 1
Stripe element 2
Stripe element 3 Stripe element 1 Duplicated
Stripe element 2 Duplicated
Stripe element 3 Duplicated
Stripe element 4 Stripe element 4 Duplicated
Segment 1
Segment 7 Segment 2
Segment 8 Segment 3
Segment 9 Segment 4
Segment 10
Segment 5
Parity (6–10)
Parity (11–15)
Parity (1–5)
Segment 6
Note: Parity is distributed across all drives in the array.
Segment 12 Segment 15 Segment 11 Segment 14 Segment 13
Segment 19
Segment 25 Segment 20
Segment 23
Segment 18
Segment 21
Segment 16
Segment 22
Segment 17
Parity (21–25)
Parity (26–30)
Parity (16–20)
Segment 24
Segment 30 Segment 27 Segment 29 Segment 26 Segment 28
Dell_PERC5_UG.book Page 14 Tuesday, February 13, 2007 6:02 PM