Hot spare
A hot spare is a disk installed in an array and kept powered and idle, ready to replace a failed disk and become the target of a rebuild without anyone swapping hardware.
The spare holds none of the array’s data while the array is healthy. When a member disk fails, the array takes the spare in and starts a rebuild onto it, which shortens the time the array spends as a degraded array.
How are spares set up?
On Linux software RAID, spares are named when the array is created, and can be added or removed later. When the driver marks a disk faulty, it starts recreating that disk’s data on a spare.
Related terms
Common misconceptions
Does a hot spare add another disk of protection?
No. The spare adds no redundancy until the rebuild onto it has finished. Until then, the array has only what its RAID level gives it, minus the failed disk.