Page 35
MegaRAID SAS Software User Guide Chapter 2: Introduction to RAID
| RAID Levels
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 two drives in a virtual drive without losing data. 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 recreate 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.
Table10 provides an overview of a RAID 6 drive group.
Figure2.5.7 shows a RAID 6 data layout. The second set of parity drives are denoted by
Q. The P drives follow the RAID 5 parity scheme.
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
Table 10: RAID 6 Overview
Uses Use 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. Can survive the loss of two drives or the loss of a drive while
another drive is being rebuilt. Provides the highest level of protection
against drive failures of all of the RAID levels. Read performance is similar to
that of RAID 5.
Weak Points Not well-suited to tasks requiring a lot of writes. A RAID 6 virtual drive has to
generate two sets of parity data for each write operation, which results in a
significant decrease in performance during writes. Drive performance is
reduced during a drive rebuild. Environments with few processes do not
perform as well because the RAID overhead is not offset by the performance
gains in handling simultaneous processes. RAID 6 costs more because of the
extra capacity required by using two parity blocks per stripe.
Drives 3 through 32
To Next Page
Loading...
