What an appropriate hot spare is

If a disk fails and no hot spare disk that exactly matches the failed disk is available, Data ONTAP uses the best available spare. Understanding how Data ONTAP chooses an appropriate spare when there is no matching spare enables you to optimize your spare allocation for your environment.

Data ONTAP picks a non-matching hot spare based on the following criteria:

If the available hot spares are not the correct size, Data ONTAP uses one that is the next size up, if there is one.
The replacement disk is downsized to match the size of the disk it is replacing; the extra capacity is not available.
If the available hot spares are not the correct speed, Data ONTAP uses one that is a different speed.
Using drives with different speeds within the same aggregate is not optimal. Replacing a disk with a slower disk can cause performance degradation, and replacing a disk with a faster disk is not cost-effective.
If the failed disk is part of a mirrored aggregate and there are no hot spares available in the correct pool, Data ONTAP uses a spare from the other pool.
Using drives from the wrong pool is not optimal because you no longer have fault isolation for your SyncMirror configuration.

If no spare exists with an equivalent disk type or checksum type, the RAID group that contains the failed disk goes into degraded mode; Data ONTAP does not combine effective disk types or checksum types within a RAID group.