User's Manual

RAID Levels

Each RAID level uses some combination of mirroring, striping, and parity to provide data redundancy or improved read and write performance. For specific
information on each RAID level, see "Choosing RAID Levels."

Organizing Data Storage for Availability and Performance

RAID provides different methods or RAID levels for organizing the disk storage. Some RAID levels maintain redundant data so that you can restore data after a
disk failure. Different RAID levels may also entail an increase or decrease in the system's I/O (read and write) performance.

Maintaining redundant data requires the use of additional physical disks. As more disks become involved, the likelihood of a disk failure increases. Because of
the differences in I/O performance and redundancy, one RAID level may be more appropriate than another based on the applications in the operating
environment and the nature of the data being stored.

When choosing a RAID level, the following performance and cost considerations apply:
l Availability or fault-tolerance. Availability or fault-tolerance refers to a system's ability to maintain operations and provide access to data even when one
of its components has failed. In RAID volumes, availability or fault-tolerance is achieved by maintaining redundant data. Redundant data includes mirrors
(duplicate data) and parity information (reconstructing data using an algorithm).
l Performance. Read and write performance can be increased or decreased depending on the RAID level you choose. Some RAID levels may be more
appropriate for particular applications.
l Cost efficiency. Maintaining the redundant data or parity information associated with RAID volumes requires additional disk space. In situations where
the data is temporary, easily reproduced, or non-essential, the increased cost of data redundancy may not be justified.
l Mean Time Between Failure (MTBF). Using additional disks to maintain data redundancy also increases the chance of disk failure at any given moment.
Although this cannot be avoided in situations where redundant data is a requirement, it does have implications for the workload of your organization's
system support staff.

For more information, see "Choosing RAID Levels."

Choosing RAID Levels

You can use RAID to control data storage on multiple disks. Each RAID level has different performance and data protection characteristics.

The following sub-sections provide specific information on how each RAID level store data as well as their performance and protection characteristics:
l "RAID Level 0 (Striping)"
l "RAID Level 1 (Mirroring)"
l "RAID Level 5 (Striping with distributed parity)"
l "RAID Level 6 (Striping with additional distributed parity)"
l "RAID Level 50 (Striping over RAID 5 sets)"
l "RAID Level 60 (Striping over RAID 6 sets)"
l "RAID Level 10 (Striping over mirror sets)"
l "RAID Level 1-Concatenated (Concatenated mirror)"
l "Comparing RAID Level Performance"

RAID Level 0 (Striping)

RAID 0 uses data striping, which is writing data in equal-sized segments across the physical disks. RAID 0 does not provide data redundancy.
Figure 5-1. Striping Disks
