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Appendix C
RAID Levels
This appendix provides a functional description of Redundant Array of Independent Disks (RAID).
This includes information about RAID and available RAID levels.
C.1 RAID Description
Redundant Array of Independent Disks (RAID) is a storage technology used to improve the
processing capability of storage systems. This technology is designed to provide reliability in disk
array systems and to take advantage of the performance gains multiple disks can offer.
RAID comes with a redundancy feature that ensures fault-tolerant, uninterrupted disk storage
operations. In the event of a disk failure, disk access will still continue normally with the failure
transparent to the host system.
RAID has six levels: RAID 0 ~ 5. RAID levels 1, 3 and 5 are the most commonly used levels, while RAID
levels 2 and 4 are rarely implemented. The following sections describe in detail each of the
commonly used RAID levels.
C.2 Non-RAID Storage
One common option for expanding disk storage capacity is simply to install multiple disk drives into
the system and then combine them end to end. This method is called disk spanning.
In disk spanning, the total disk capacity is equivalent to the sum of the capacities of all SCSI drives
in the combination. This combination appears to the system as a single logical drive. Thus,
combining four 1GB SCSI drives in this way, for example, would create a single logical drive with a
total disk capacity of 4GB.
Disk spanning is considered non-RAID due to the fact that it provides neither redundancy nor
improved performance. Disk spanning is inexpensive, flexible, and easy to implement; however, it
does not improve the performance of the drives and any single disk failure will result in total data
loss.
RAID Levels
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