Users Guide

Understanding RAID concepts
Storage Management uses the Redundant Array of Independent Disks (RAID) technology to provide Storage Management capability.
Understanding Storage Management requires an understanding of RAID concepts, as well as some familiarity with how the RAID controllers
and operating system view disk space on your system.
Topics:
• RAID
• Organizing Data Storage For Availability And Performance
• Choosing RAID Levels And Concatenation
• Comparing RAID Level And Concatenation Performance
• No-RAID
RAID
RAID is a technology for managing the storage of data on the physical disks that reside or are attached to the system. A key aspect of
RAID is the ability to span physical disks so that the combined storage capacity of multiple physical disks can be treated as a single,
extended disk space. Another key aspect of RAID is the ability to maintain redundant data which can be used to restore data in the event of
a disk failure. RAID uses dierent techniques, such as striping, mirroring, and parity, to store and reconstruct data. There are dierent RAID
levels that use dierent methods for storing and reconstructing data. The RAID levels have dierent characteristics in terms of read/write
performance, data protection, and storage capacity. Not all RAID levels maintain redundant data, which means for some RAID levels lost
data cannot be restored. The RAID level you choose depends on whether your priority is performance, protection, or storage capacity.
NOTE
: The RAID Advisory Board (RAB) denes the specications used to implement RAID. Although RAB denes the RAID
levels, commercial implementation of RAID levels by dierent vendors may vary from the actual RAID specications. An
implementation of a particular vendor may aect the read and write performance and the degree of data redundancy.
Hardware and software RAID
RAID can be implemented with either hardware or software. A system using hardware RAID has a RAID controller that implements the RAID
levels and processes data reads and writes to the physical disks. When using software RAID provided by the operating system, the
operating system implements the RAID levels. For this reason, using software RAID by itself can slow the system performance. You can,
however, use software RAID along with hardware RAID volumes to provide better performance and variety in the conguration of RAID
volumes. For example, you can mirror a pair of hardware RAID 5 volumes across two RAID controllers to provide RAID controller
redundancy.
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 dierent physical disks. If one of the disks in the mirror fails, the system can continue to operate using the
unaected 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.
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16 Understanding RAID concepts