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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 gure shows an example of disk mirroring.
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 1 MB of drive
space and has 64 KB of data residing on each drive in the stripe. In this case, the stripe size is 1 MB and
the strip size is 64 KB.
Strip Size
The strip size is the portion of a stripe that resides on a single drive.
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 a RAID drive
group, 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.
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