RAID Glossary

Short for Redundant Array of Independent (or Inexpensive) Disks, a category of disk drives that employ two or more drives in combination for fault tolerance and performance. RAID disk drives are used frequently on servers but aren't generally necessary for personal computers.

There are number of different RAID levels. The three most common are 0, 3, and 5:

* Level 0: Provides data striping (spreading out blocks of each file across multiple disks) but no redundancy. This improves performance but does not deliver fault tolerance.

* Level 1: Provides disk mirroring.

* Level 3: Same as Level 0, but also reserves one dedicated disk for error correction data. It provides good performance and some level of fault tolerance.

* Level 5: Provides data striping at the byte level and also stripe error correction information. This results in excellent performance and good fault tolerance.

The level of a RAID system (Redundant Array of Inexpensive Disks) relates to its operating mode and how the hard disks are combined to form a single logical drive.


RAID Level 0 requires a minimum of 2 drives to implement

RAID 0 implements a striped disk array, the data is broken down into blocks and each block is written to a separate disk drive I/O performance is greatly improved by spreading the I/O load across many channels and drives Best performance is achieved when data is striped across multiple controllers with only one drive per controller This is of particular benefit for video editing and image editing programs. However, if one drive fails, you lose the data on all drives.


For Highest performance, the controller must be able to perform two concurrent separate Reads per mirrored pair or two duplicate Writes per mirrored pair. RAID Level 1 requires a minimum of 2 drives to implement

RAID-1 (mirroring) involves mirroring the complete contents of one hard disk onto another. From the security standpoint, this is ideal (although not cheap), as the redundancy uses 50 percent of your hard disk capacity. There is a marginal increase in performance. Write access is slightly slower, but the controller uses the fastest available disk for reading data. Twice the Read transaction rate of single disks, same Write transaction rate as single disks. 100% redundancy of data means no rebuild is necessary in case of a disk failure, just a copy to the replacement disk. Transfer rate per block is equal to that of a single disk.


RAID-3 requires at least three hard disks, one of which is used to store error-correction data. Should a disk fail, the missing data can be restored from the parity and error-detection information on the redundant disk using an allocation algorithm. As RAID-3 interleaves the data across the drives, read speed is good, but writing is slower.


Each entire data block is written on a data disk; parity for blocks in the same rank is generated on Writes, recorded in a distributed location and checked on Reads. RAID Level 5 requires a minimum of 3 drives to implement

RAID-5 writes the original data and the error-correction information across all available drives (at least three). Writing is thus spread across all the drives in a RAID-5 system, which means that both reading and writing operations may overlap. RAID-5 offers good price/ performance, as only the space equivalent to one disk is allocated to the redundant data. Highest Read data transaction rate. Medium Write data transaction rate. Low ratio of ECC (Parity) disks to data disks means high efficiency

RAID 0+1

RAID Level 0+1 requires a minimum of 4 drives to implement.

RAID 0+1 is a combination of RAID level 0 and level 1. The disks are combined using the striping technique, and then mirrored. This only makes sense if speed is a priority, since two mirrored drives represent an expensive form of insurance. RAID 0+1 is implemented as a mirrored array whose segments are RAID 0 arrays. RAID 0+1 has the same fault tolerance as RAID level 5. RAID 0+1 has the same overhead for fault-tolerance as mirroring alone. High I/O rates are achieved thanks to multiple stripe segments. Excellent solution for sites that need high performance but are not concerned with achieving maximum reliability.


RAID Level 10 requires a minimum of 4 drives to implement.

RAID 10 is implemented as a striped array whose segments are RAID 1 arrays. RAID 10 has the same fault tolerance as RAID level 1. RAID 10 has the same overhead for fault-tolerance as mirroring alone. High I/O rates are achieved by striping RAID 1 segments. Under certain circumstances, RAID 10 array can sustain multiple simultaneous drive failures. Excellent solution for sites that would have otherwise gone with RAID 1 but need some additional performance boost