Avago Technologies MegaRAID Fast Path Software - Manual (Page 26)

Page 26 LSI Corporation Confidential | July 2011
MegaRAID SAS Software User Guide Chapter 2: Introduction to RAID | Components and Features
2.4.9 Disk Mirroring With mirroring (used in RAID 1 and RAID 10), data written to one drive is simultaneously
written to another drive. The primary advantage of disk mirroring is that it provides 100
percent data redundancy. Because the contents of the disk are completely written to a
second disk, data is not lost if one disk fails. In addition, both drives contain the same
data at all times, so either disk can act as the operational disk. If one disk fails, the
contents of the other disk can be used to run the system and reconstruct the failed disk.
Disk mirroring provides 100 percent redundancy, but it is expensive because each drive
in the system must be duplicated.
Figure 4 shows an example of disk mirroring.
Figure 4: Example of Disk Mirroring (RAID 1)
2.4.10 Parity 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 .
Segment 1
Segment 2
Segment 3 Segment 1 Duplicated
Segment 2 Duplicated
Segment 3 Duplicated
Segment 4 Segment 4 Duplicated
Table 2: Types of Parity
Parity Type Description
Dedicated The parity data on two or more drives is stored on an additional disk.
Distributed The parity data is distributed across more than one drive in the system.