Users Guide

• 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 xed-size units to each physical disk in the virtual disk using a sequential pattern. For example, if the
virtual disk includes ve physical disks, the stripe writes data to physical disks one through ve 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 specic information on each RAID level, see Choosing RAID Levels And Concatenation.
Organizing Data Storage For Availability And
Performance
RAID provides dierent 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. Dierent 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 dierences 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 eciency — 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 justied.
• 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 sta 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 dierent performance
and data protection characteristics.
Understanding RAID concepts
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