Software Support

same data always. Either side of the mirror can act as the operational side. A mirrored RAID disk group is
comparable in performance to a RAID 5 disk group in read operations but faster in write operations.
• Striping — Disk striping writes data across all physical disks in a virtual disk. Each stripe consists of
consecutive virtual disk data addresses that are mapped in fixed-size units to each physical disk in the virtual
disk using a sequential pattern. For example, if the virtual disk includes five physical disks, the stripe writes data
to physical disks one through five without repeating any of the physical disks. The amount of space consumed
by a stripe is the same on each physical disk. The portion of a stripe that resides on a physical disk is a stripe
element. Striping by itself does not provide data redundancy. Striping in combination with parity does provide
data redundancy.
• 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
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