Users Guide

● Stripe size — The total disk space consumed by a stripe not including a parity disk. For example, consider a stripe that
contains 64KB of disk space and has 16KB of data residing on each disk in the stripe. In this case, the stripe size is 64KB and
the stripe element size is 16KB.
● Stripe element — A stripe element is the portion of a stripe that resides on a single physical disk.
● Stripe element size — The amount of disk space consumed by a stripe element. For example, consider a stripe that contains
64KB of disk space and has 16KB of data residing on each disk in the stripe. In this case, the stripe element size is 16KB and
the stripe size is 64KB.
● Parity — Parity refers to redundant data that is maintained using an algorithm in combination with striping. When one of the
striped disks fails, the data can be reconstructed from the parity information using the algorithm.
● Span — A span is a RAID technique used to combine storage space from groups of physical disks into a RAID 10, 50, or 60
virtual disk.
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 And Concatenation.
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 also entail an increase or decrease in the I/O (read and write)
performance of a system.
Maintaining redundant data requires the use of additional physical disks. The possibility of a disk failure increases with an
increase in the number of disks. Since 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 concatenation or a RAID level, the following performance and cost considerations apply:
● Availability or fault-tolerance — Availability or fault-tolerance refers to the ability of a system 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).
● 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.
● 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.
● 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 option cannot be avoided in situations where redundant data is a requirement, it
does have implications on the workload of the system support staff within your organization.
● Volume — Volume refers to a single disk non-RAID virtual disk. You can create volumes using external utilities like the
O-ROM <Ctrl> <r>. Storage Management does not support the creation of volumes. However, you can view volumes and
use drives from these volumes for creation of new virtual disks or Online Capacity Expansion (OCE) of existing virtual disks,
provided free space is available. Storage Management allows Rename and Delete operations on such volumes.
Choosing RAID Levels And Concatenation
You can use RAID or concatenation to control data storage on multiple disks. Each RAID level or concatenation has different
performance and data protection characteristics.
The following topics provide specific information on how each RAID level or concatenation store data as well as their
performance and protection characteristics:
● Concatenation
● RAID Level 0 (Striping)
● RAID Level 1 (Mirroring)
● RAID Level 5 (Striping With Distributed Parity)
Understanding RAID concepts
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