User's Manual

RAID Concepts

RAID uses particular techniques for writing data to disks. These techniques enable RAID to provide data

redundancy or better performance. These techniques include:

• Mirroring — Duplicating data from one physical disk to another physical disk. Mirroring provides data

redundancy by maintaining two copies of the same data on different physical disks. If one of the disks

in the mirror fails, the system can continue to operate using the unaffected disk. Both sides of the

mirror contain the 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