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RAID levels that include redundancy allow assigning more disks than required to an array. The extra disks are used as spares when one of the main disks fails. For instance, in a mirror of two disks plus one spare, if one of the first two disks fails, the kernel will automatically (and immediately) reconstruct the mirror using the spare disk, so that redundancy stays assured after the reconstruction time. This can be used as another kind of safeguard for critical data.
One would be forgiven for wondering how this is better than simply mirroring on three disks to start with. The advantage of the “spare disk” configuration is that the spare disk can be shared across several RAID volumes. For instance, one can have three mirrored volumes, with redundancy assured even in the event of one disk failure, with only seven disks (three pairs, plus one shared spare), instead of the nine disks that would be required by three triplets.
This RAID level, although expensive (since only half of the physical storage space, at best, is useful), is widely used in practice. It is simple to understand, and it allows very simple backups: since both disks have identical contents, one of them can be temporarily extracted with no impact on the working system. Read performance is often increased since the kernel can read half of the data on each disk in parallel, while write performance isn't too severely degraded. In case of a RAID-1 array of N disks, the data stays available even with N-1 disk failures.
<emphasis>CAUTION</emphasis> RAID is not Backup
<primary>backup</primary>
RAID systems are not backup mechanisms. While RAID increases the redundancy - and therefore the availability of a system - and protects against disk failures, backups are done to protect data from being altered, deleted, getting corrupted, etc., and to be able to restore them if necessary. To demonstrate this: If you remove one or all files by accident, a RAID will mirror this change, but it will not provide the means to restore the file(s). So while there is clearly an overlap, they are not the same and should be used in conjunction with each other.
RAID-4
This RAID level, not widely used, uses N disks to store useful data, and an extra disk to store redundancy information. If that disk fails, the system can reconstruct its contents from the other N. If one of the N data disks fails, the remaining N-1 combined with the “parity” disk contain enough information to reconstruct the required data.
<primary>RAID</primary><secondary>4</secondary>
<primary>RAID</primary><secondary>parity</secondary>
RAID-4 isn't too expensive since it only involves a one-in-N increase in costs and has no noticeable impact on read performance, but writes are slowed down. Furthermore, since a write to any of the N disks also involves a write to the parity disk, the latter sees many more writes than the former, and its lifespan can shorten dramatically as a consequence. Data on a RAID-4 array is safe only up to one failed disk (of the N+1).

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  1. Debian Handbook
  2. 12_advanced-administration
  3. English
RAID systems are not backup mechanisms. While RAID increases the redundancy - and therefore the availability of a system - and protects against disk failures, backups are done to protect data from being altered, deleted, getting corrupted, etc., and to be able to restore them if necessary. To demonstrate this: If you remove one or all files by accident, a RAID will mirror this change, but it will not provide the means to restore the file(s). So while there is clearly an overlap, they are not the same and should be used in conjunction with each other.
a year ago
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