A hot spare is a slice (not a volume) that is functional and available, but not in use. A hot spare is reserved, meaning that it stands ready to substitute for a failed slice in a submirror or RAID 5 volume.
Hot spares provide protection from hardware failure because slices from RAID 1 or RAID 5 volumes are automatically replaced and resynchronized when they fail. The hot spare can be used temporarily until a failed submirror or RAID 5 volume slice can be either fixed or replaced.
You create hot spares within hot spare pools. Individual hot spares can be included in one or more hot spare pools. For example, you might have two submirrors and two hot spares. The hot spares can be arranged as two hot spare pools, with each pool having the two hot spares in a different order of preference. This strategy enables you to specify which hot spare is used first, and it improves availability by having more hot spares available.
A submirror or RAID 5 volume can use only a hot spare whose size is equal to or greater than the size of the failed slice in the submirror or RAID 5 volume. If, for example, you have a submirror made of 1 Gbyte drives, a hot spare for the submirror must be 1 Gbyte or greater.