If an I/O error occurs in a disk that is connected to a lower level group, a spare disk is automatically connected to the highest level mirror group, but not to the lower level group. Therefore, the capacity of the spare disk must be larger than the available size of the lower group.
Figure 3.5 Hot Spare for Hierarchized Mirror Groups
A spare disk is selected independently of the disk case or controller number of a disk where an I/O error occurred.
In external hot spare mode (default), spare disks are selected randomly.
In internal hot spare mode, spare disks whose controller number is 0 and that do not belong to disk array units are selected.
