HP-UX System Administrator's Guide: Routine Management Tasks

continue to work without interruption. The bad disk can be replaced at a more
convenient time when the system can be brought down without causing problems.
Once the system is rebooted, the mirroring software will cause the mirrored data to be
copied back to the replacement disk and the process of mirroring will begin again.
With three-way disk mirroring, two copies of each disk’s data are maintained. This
strategy is even more robust than two-way mirroring which is described above and it
eliminates the need to bring the system down at all in order to replace a bad disk.
NOTE: Using Version 1 LVM Volume Groups, you can have up to three copies of
your data (the original plus two mirror copies). Using Version 2 LVM Volume Groups
you can have up to six copies of your data (the original plus five mirror copies).
To use disk mirroring, you will need to use LVM or VxVM as your disk management
strategy and (if you are using LVM) have available the MirrorDisk/UX software product.
MirrorDisk/UX causes every write to the original volume to also be written to the copy
or copies of the original volume. Note that the original data and its copied data may
be spread over more than one disk.
The main advantage of software mirroring over hardware mirroring, which is discussed
in “Using Disk Arrays” (page 206), is that the cost of implementation is lower. The main
disadvantage of software mirroring relates to its increased complexity of management.
That is, it will probably be significantly more difficult to manage a system with a large
number of disks as compared to a system with a single disk array.
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.
Disk Arrays Using RAID Data Protection Strategies
RAID stands for Redundant Arrays of Independent Disks. Various configurations or
RAID levels are available. We will mention several.
206 Using High Availability Strategies