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* '''Redundant array of independent disks''' ('''[[RAID]]''') – This method generalizes the device mirroring above by allowing one device in a group of devices to fail and be replaced with the content restored (Device mirroring is RAID with ''n=2''). RAID groups of ''n=5'' or ''n=6'' are common. ''n>2'' saves storage, when compared with ''n=2'', at the cost of more processing during both regular operation (with often reduced performance) and defective device replacement.
Device mirroring and typical RAID are [[designed]] to handle a single device failure in the RAID group of devices. However, if a second failure occurs before the RAID group is completely repaired from the first failure, then data can be lost. The probability of a single failure is typically small. Thus the probability of two failures in the same RAID group in time proximity is much smaller (approximately the probability squared, i.e., multiplied by itself). If a database cannot tolerate even such a smaller probability of data loss, then the RAID group itself is replicated (mirrored). In many cases such mirroring is done geographically remotely, in a different storage array, to handle recovery from disasters (see disaster recovery above).
=== Network connectivity ===
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