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

Mirroring (RAID Level 1)
In a RAID 1 configuration, all data is duplicated on two or more disks.
In hardware mirroring, each disk has a “twin,” a backup disk containing an exact copy
of its data. Some RAID 1 implementations duplicate not only the disks but the array
controller and the power supply as well.
In the case of software mirroring (discussed in “Using Software Mirroring as a Disk
Protection Strategy” (page 205)), the original data and its copied data may be spread
over more than one disk as a result of using LVM or VxVM software to manage your
disk storage.
Pros and Cons
If a disk fails, the array controller will automatically switch all system I/O activity to
the drive containing the copy. This prevents the system from going down in the event
a drive fails. The disadvantage of hardware mirroring is the expense of duplicating
your hardware.
Recommended Uses and Performance Considerations
Use when high data availability is required. Can provide up to twice the read I/0 rate
although writes are similar to using single disks. The data transfer rate is similar to
using single disks.
Disk Striping (RAID Level 0)
This configuration interleaves data in blocks across multiple disks.
Pros and Cons
RAID 0 offers increased performance because several I/O transfers can be done at the
same time. However, it does not provide data redundancy in the event of disk failure.
Recommended Uses and Performance Considerations
Effective for high performance I/O environments using noncritical data.
Data striping can also prevent “hot spots,” which are caused by constant hits on a single
drive; a specific drive may be accessed so often that it will slow down I/O traffic, or
shorten the life of the drive.
RAID 3
This type of array uses a separate data protection disk to store encoded data. RAID 3
is designed to provide a high transfer rate.
RAID 3 organizes data by segmenting a user data record into either bit- or byte-sized
chunks and evenly spreading the data across N drives in parallel. One of the drives acts
as a parity drive. In this manner, every record that is accessed is delivered at the full
media rate of the N drives that comprise the stripe group. The drawback is that every
record I/O stripe accesses every drive in the group.
Disk Arrays Using RAID Data Protection Strategies 207