Data redundancy is necessary to prevent instances in which
a single disk failure can cause a system to go down until the problem
is located and corrected. There are two methods of providing data
redundancy: software mirroring and hardware mirroring. Each represents
RAID Level 1. (See “Using Disk Arrays” for
more information on the meaning of the various RAID levels.)
Software mirroring allows you to maintain
identical copies of your data (except for the root disk), so that
each set of data has, in effect, a perfect clone of itself. In the
event a disk fails, the system can use the mirrored copy of the
data, thus allowing users to 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.
To use these types of disk mirroring, you will need to use
LVM or VxVM as your disk management strategy and have available
the MirrorDisk/UX software product. MirrorDisk/UX causes every write
to the original disk to also be written to the copy or copies of
the original disk. 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”,
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.
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