Avago Technologies MegaRAID Fast Path Software - Manual (Page 35)

LSI Corporation Confidential | July 2011 Page 35
MegaRAID SAS Software User Guide Chapter 2: Introduction to RAID | RAID Levels
Table 9 provides an overview of RAID 5. Figure 9 provides a graphic example of a RAID 5
drive group.
Figure 9: RAID 5 Drive Group with Six Drives
2.5.6 RAID 6 RAID 6 is similar to RAID 5 (disk striping and parity), except that instead of one parity
block per stripe, there are two. With two independent parity blocks, RAID 6 can survive
the loss of any two drives in a virtual drive without losing data. RAID 6 provides a high
level of data protection through the use of a second parity block in each stripe. Use
RAID 6 for data that requires a very high level of protection from loss.
In the case of a failure of one drive or two drives in a virtual drive, the RAID controller
uses the parity blocks to re-create all of the missing information. If two drives in a RAID
6 virtual drive fail, two drive rebuilds are required, one for each drive. These rebuilds do
not occur at the same time. The controller rebuilds one failed drive, and then the other
failed drive.
Table 9: RAID 5 Overview
Uses Provides high data throughput, especially for large files. Use RAID 5 for
transaction processing applications because each drive can read and write
independently. If a drive fails, the RAID controller uses the parity drive to
re-create all missing information. Use also for office automation and online
customer service that requires fault tolerance. Use for any application that
has high read request rates but low write request rates.
Strong points Provides data redundancy, high read rates, and good performance in most
environments. Provides redundancy with lowest loss of capacity.
Weak points Not well-suited to tasks requiring lot of writes. Suffers more impact if no
cache is used (clustering). Drive performance is reduced if a drive is being
rebuilt. Environments with few processes do not perform as well because
the RAID overhead is not offset by the performance gains in handling
simultaneous processes.
Number of Drives
in this RAID Level 3 through 32
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 drive group.
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