A hot spare is a slice (not a volume) that is running but not in use. It is reserved, meaning that the hot spare stands ready to substitute for an errored slice in a submirror or RAID 5 volume.
Because slice replacement and the resyncing of failed slices is automatic, hot spares provide protection from hardware failure. The hot spare can be used temporarily until a failed submirror or RAID 5 volume slice is either fixed or replaced.
Hot spares remain idle most of the time, and so do not contribute to normal system operation. In addition, slices designated as hot spares cannot be used in any other metadevice, nor can they be used to hold data while idle.
You create hot spares within hot spare pools. Individual hot spares can be included in one or more hot spare pools. For example, you may 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 enables you to specify which hot spare is used first. It also improves availability by having more hot spares available.
You cannot use hot spares within other metadevices, for example within a submirror. They must remain ready for immediate use in the event of a slice failure. A hot spare must be a physical slice. It cannot be a volume. In addition, hot spares cannot be used to hold state database replicas.
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.
Note Only devices with replicated data (RAID 1 and RAID 5 devices) can use hot spares. When a hot spare takes over, any data on the failed slice must be recreated from the existing replicated data.