Users Guide

• 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.
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 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.
Choosing RAID levels
You can use RAID to control data storage on multiple disks. Each RAID level or concatenation has dierent performance and data
protection characteristics.
NOTE
: The H3xx PERC controllers do not support RAID levels 6 and 60.
The following topics provide specic information on how each RAID level store data as well as their performance and protection
characteristics:
• Raid level 0 (striping)
• Raid level 1 (mirroring)
• Raid level 5 (striping with distributed parity)
• Raid level 6 (striping with additional distributed parity)
• Raid level 50 (striping over raid 5 sets)
• Raid level 60 (striping over raid 6 sets)
• Raid level 10 (striping over mirror sets)
Managing storage devices
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