User's Manual

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 concatenation or 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.
l 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.
For more information, see Choosing RAID Levels and Concatenation.
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 sub-sections provide specific information on how each RAID level or concatenation store data as well as their performance and protection
characteristics:
l Concatenation
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 and Concatenation Performance
l No-RAID
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Concatenation
In Storage Management, concatenation refers to storing data on either one physical disk or on disk space that spans multiple physical disks. When spanning
more than one disk, concatenation enables the operating system to view multiple physical disks as a single disk.
Data stored on a single disk can be considered a simple volume. This disk could also be defined as a virtual disk that comprises only a single physical disk.
Data that spans more than one physical disk can be considered a spanned volume. Multiple concatenated disks can also be defined as a virtual disk that
comprises more than one physical disk.
A dynamic volume that spans to separate areas of the same disk is also considered concatenated.
When a physical disk in a concatenated or spanned volume fails, the entire volume becomes unavailable. Because the data is not redundant, it cannot be
restored by rebuilding from a mirrored disk or parity information. Restoring from a backup is the only option.
Because concatenated volumes do not use disk space to maintain redundant data, they are more cost-efficient than volumes that use mirrors or parity
information. A concatenated volume may be a good choice for data that is temporary, easily reproduced, or that does not justify the cost of data redundancy.
In addition, a concatenated volume can easily be expanded by adding an additional physical disk.
Figure 3-1. Concatenating Disks