Maintaining the proper number of spares for disks in multi-disk carriers is critical for optimizing storage redundancy and minimizing the amount of time Data ONTAP must spend copying disks to achieve an optimal disk layout.
You must maintain a minimum of two hot spares for multi-disk carrier disks at all times. To support the use of the Maintenance Center, and to avoid issues caused by multiple concurrent disk failures, you should maintain at least four hot spares for steady state operation, and replace failed disks promptly.
If two disks fail at the same time with only two available hot spares, Data ONTAP might not be able to swap the contents of both the failed disk and its carrier mate to the spare disks. This scenario is called a stalemate. If this happens, you are notified through EMS messages and AutoSupport messages. When the replacement carriers become available, you must follow the instructions provided by the EMS messages or contact technical support to recover from the stalemate.