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12Gb/s MegaRAID SAS Software User Guide
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March 2014
Chapter 2: Introduction to RAID
Components and Features
Stripe Width
Stripe width is the number of drives involved in a drive group where striping is implemented. For example, a four-disk
drive group with disk striping has a stripe width of four.
Stripe Size
The stripe size is the length of the interleaved data segments that the RAID controller writes across multiple drives, not
including parity drives. For example, consider a stripe that contains 64 KB of disk space and has 16 KB of data residing
on each disk in the stripe. In this case, the stripe size is 64 KB, and the strip size is 16 KB.
Strip Size
The strip size is the portion of a stripe that resides on a single drive.
2.1.9 Disk Mirroring
With disk 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 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. The following figure shows an example of disk mirroring.
Figure 4 Example of Disk Mirroring (RAID 1)
2.1.10 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 the following table.
Table 2 Types of Parity
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 the following figure. RAID 5 uses parity
to provide redundancy for one drive failure without duplicating the contents of entire drives. RAID 6 also uses
distributed parity and disk striping, but adds a second set of parity data so that it can survive up to two drive failures.
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.
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