Having insufficient spares increases the risk of a disk failure with no available spare, resulting in a degraded RAID group. A spare disk is also required to provide important information (a core file) to technical support in case of a controller disruption.
MSATA disks, or disks in a multi-disk carrier, should have four hot spares during steady state operation, and you should never allow the number of MSATA hot spares to dip below two.
For RAID groups composed of SSDs, you should have at least one spare disk.
For all other Data ONTAP disk types, you should have at least one matching or appropriate hot spare available for each kind of disk installed in your storage system. However, having two available hot spares for all disks provides the best protection against disk failure. Having at least two available hot spares provides the following benefits:
Data ONTAP uses the maintenance center to test suspect disks and take offline any disk that shows problems.
A single spare disk can serve as a hot spare for multiple RAID groups. However, if any disk in those RAID groups fails, then no spare is available for any future disk failures, or for a core file, until the spare is replaced. For this reason, having more than one spare is recommended.