A hot spare is a slice (not a metadevice) 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 RAID5 metadevice.
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 RAID5 metadevice 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 metadevice. In addition, hot spares cannot be used to hold state database replicas.
A submirror or RAID5 metadevice can use only a hot spare whose size is equal to or greater than the size of the failed slice in the submirror or RAID5 metadevice. If, for example, you have a submirror made of 1 Gbyte drives, a hot spare for the submirror must be 1 Gbyte or greater.
A prerequisite for hot spares is that the metadevices with which they are associated have replicated data. When a hot spare takes over, any data on the failed slice must be recreated. For this reason, only mirrors and RAID5 metadevices use hot spares.