Users Guide
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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.
Related links
What is RAID?
Organizing Data Storage For Availability And Performance
Choosing RAID Levels And Concatenation
Comparing RAID Level And Concatenation Performance
What is 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 dierent techniques, such as striping, mirroring, and parity, to store and reconstruct data.
There are dierent RAID levels that use dierent methods for storing and reconstructing data. The RAID levels have dierent
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) denes the specications used to implement RAID. Although RAB denes the
RAID levels, commercial implementation of RAID levels by dierent vendors may vary from the actual RAID
specications. An implementation of a particular vendor may aect 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 conguration 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 dierent physical disks. If one of the disks in the mirror fails, the system can continue to operate
using the unaected 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 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
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