Parity - Lenovo ThinkServer RD330 Software User's Manual

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Chapter 2: Introduction to RAID Components and Features
2.4.10

Parity

Page 26
Disk mirroring provides 100 percent redundancy, but is expensive because each drive
in the system must be duplicated.
Figure 4:
Parity generates a set of redundancy data from two or more parent data sets. The
redundancy data can be used to reconstruct one of the parent data sets in the event of
a drive failure. Parity data does not fully duplicate the parent data sets, but parity
generation can slow the write process. In RAID, this method is applied to entire drives
or stripes across all of the drives in a drive group. The types of parity are described in
Table 2.
Table 2: Types of Parity
Parity Type
Dedicated
Distributed
RAID 5 combines distributed parity with disk striping. If a single drive fails, it can be
rebuilt from the parity and the data on the remaining drives. An example of a RAID 5
drive group is shown in Figure 5. RAID 5 uses parity to provide redundancy for one
drive failure without duplicating the contents of entire drives. RAID 6 uses distributed
parity and disk striping, also, but adds a second set of parity data so that it can survive
up to two drive failures.
Segment 1 Segment 2
Segment 7 Segment 8
Segment 13 Segment 14
Segment 19 Segment 20
Segment 25
Parity (21-25)
Segment 26
Parity (26–30)
Note: Parity is distributed across all drives in the drive group.
Figure 5:
Figure 4
Segment 1
Segment 2
Segment 3
Segment 4
Example of Disk Mirroring (RAID 1)
The parity data on two or more drives is stored on an additional disk.
The parity data is distributed across more than one drive in the system.
Segment 3
Segment 9
Segment 15
Parity (16-20)
Segment 21
Segment 27
Example of Distributed Parity (RAID 5)
MegaRAID SAS Software User Guide
shows an example of disk mirroring.
Segment 1 Duplicated
Segment 2 Duplicated
Segment 3 Duplicated
Segment 4 Duplicated
Description
Segment 4 Segment 5
Segment 10 Parity (6-10)
Segment 11
Parity (11–15)
Segment 16 Segment 17
Segment 22 Segment 23
Segment 28
Segment 29
Parity (1-5)
Segment 6
Segment 12
Segment 18
Segment 24
Segment 30