Avago Technologies MegaRAID Fast Path Software - Manual (Page 27)

LSI Corporation Confidential | July 2011 Page 27
MegaRAID SAS Software User Guide Chapter 2: Introduction to RAID | Components and Features
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.
Figure 5: Example of Distributed Parity (RAID 5)
2.4.11 Disk Spanning Disk spanning allows multiple drives to function like one big drive. Spanning
overcomes lack of disk space and simplifies storage management by combining
existing resources or adding relatively inexpensive resources. For example, four 20-GB
drives can be combined to appear to the operating system as a single 80-GB drive.
Spanning alone does not provide reliability or performance enhancements. Spanned
virtual drives must have the same stripe size and must be contiguous. In
Figure 6 , RAID
1 drive groups are turned into a RAID 10 drive group.
NOTE: Make sure that the spans are in different backplanes, so that if one span fails, you
do not lose the whole drive group.
Figure 6: Example of Disk Spanning
Spanning two contiguous RAID 0 virtual drives does not produce a new RAID level or
add fault tolerance. It does increase the capacity of the virtual drive and improves
performance by doubling the number of spindles.
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
60 GB 60 GB
Can Be Accessed as
One 120-GB Drive
60 GB 60 GB
Can Be Accessed as
One 120-GB Drive