What is the function of hot spare in storage?

In storage, a hot spare is a spare disk that is pre-configured and ready to be used to rebuild data in the event of a disk failure in a RAID array. When a disk failure occurs in a RAID array, the data that was stored on the failed disk is rebuilt using the data from the remaining disks and the hot spare. The rebuild process is done automatically by the RAID controller, and it is done in a way that does not disrupt the normal operation of the RAID array. The hot spare disk is usually kept in a powered-on and ready state, so that it can be used immediately in case of a disk failure. It is also known as "hot spare disk" or "hot spare drive".