Managing Systems and Workgroups: A Guide for HP-UX System Administrators

Using High Availability Strategies
Using Disk Arrays
Appendix A 949
Using Disk Arrays
A disk array consists of multiple disk drives under the command of an
array controller. The disk array incorporates features that differentiate
it from traditional disk storage devices.
Most types of disk arrays provide for one of two possible options for
protecting data in the event of a disk failure. This becomes more and
more important as the number of disks on a system increases, since the
chance of a disk failure also increases. Normally, a disk crash brings the
system down or prevents access to data, removing it from service until
the problem is located and repaired, and the data is reloaded.
The first kind of data protection is called data encoding. When a disk
drive fails, the array controller generates encoded data, which is similar
to parity or checksum calculations. This allows missing user data to be
reconstructed using a mathematical formula to rebuild lost data. As a
result, the data remains accessible and the system remains up and
running without suffering any downtime.
The second method of data protection utilizes hardware mirroring as
a means of providing high data availability by duplicating data on
redundant disk drives. As a result, failure in one disk still allows access
to the data on an alternate disk.