Information Technology Reference
In-Depth Information
striped virtual device can be accessed at the same time. In addition, like con-
catenated virtual devices, it provides means for using slices that might other-
wise be too small for use.
As with concatenated virtual devices, multiple disks increase the chance that
a failure of any one disk will result in the failure of the virtual device.
A concatenated striped virtual device is a striped virtual device that has been
expanded by concatenating additional slices to the end of the device.
Mirroring and Duplexing
Mirroring
is the technique of copying data being written to an online device to
an offline device. This provides a real-time backup of data that can be brought
online to replace the original device in the event that the original device fails.
Typically, the two disks used in this way share the same controller.
Duplexing
is similar to mirroring, except that each disk has its own controller.
This provides a little more redundancy by eliminating the controller as a sin-
gle point of failure.
RAID Configurations
One approach to improving data availability is to arrange disks in various con-
figurations, known as
Redundant Arrays of Inexpensive Disks (RAID)
. Table 14.1
lists the levels of RAID.
Table 14.1
RAID Levels
Level
Description
0
Striping or concatenation
1
Mirroring and duplexing
0+1
Striping then mirroring (striped disks that are mirrored)
2
Hamming Error Code Correction (ECC), used to detect and correct errors
3
Bit-interleaved striping with parity information (separate disk for parity)
4
Block-interleaved striping with parity information (separate disk for parity)
5
Block-interleaved striping with distributed parity information
6
Block-interleaved striping with two independent distributed parity schemes
7
Block-interleaved striping with asynchronous I/O (input/output) transfers
and distributed parity information
1+0
Mirroring then Striping (mirrored disks that are striped)
