Some hardware RAID implementations permit hot swapping; that is, faulty
disks can be removed and replaced by new ones without turning power off. Hot
swapping reduces the mean time to repair, since replacement of a disk does not
have to wait until a time when the system can be shut down. In fact many critical
systems today run on a 24 × 7 schedule; that is, they run 24 hours a day, 7 days a
week, providing no time for shutting down and replacing a failed disk. Further,
many RAID implementations assign a spare disk for each array (or for a set of disk
arrays). If a disk fails, the spare disk is immediately used as a replacement. As a
result, the mean time to repair is reduced greatly, minimizing the chance of any
data loss. The failed disk can be replaced at leisure.
The power supply, or the disk controller, or even the system interconnection
in a RAID system could become a single point of failure that could stop functioning
of the RAID system. To avoid this possibility, good RAID implementations have
multiple redundant power supplies (with battery backups so they continue to
function even if power fails). Such RAID systems have multiple disk interfaces,
andmultiple interconnections to connect the RAID system to the computer system
(or to a network of computer systems). Thus, failure of any single component will
not stop the functioning of the RAID system.