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[ The PC Guide | Systems and Components Reference Guide | Hard Disk Drives | Hard Disk Performance, Quality and Reliability | Redundant Arrays of Inexpensive Disks (RAID) | RAID Levels | Technical Factors Differentiating RAID Levels ]
Degradation and Rebuilding
A fault-tolerant (redundant) array can continue operating after experiencing a failure of one (or sometimes more) of its drives. When this happens the array enters a degraded state until the faulty drive is replaced and its contents are rebuilt. Running in a degraded state has an important impact on both the performance and subsequent fault tolerance of the array; see this full discussion for more.
Some RAID levels handle degradation more "elegantly" than others; some can in fact tolerate the loss of more than one drive, while others cannot handle the loss of even one. While in a degraded mode, some RAID levels suffer much more performance loss than others; generally, the more complex the RAID level, the more its performance decreases when the array is degraded (simple mirroring shows little loss of performance, while striping with parity shows much more). Also, alternate ways of implementing multiple RAID levels can be surprisingly different in terms of the number of lost drives they can tolerate, and also in the impact on performance when rebuilding after a fault.