Using Hot Spared Disks

A hot spared disk drive is a disk that is reserved for swapping with a bad disk that has no mirrored or parity data. It is simply a spare disk that is online and waiting for a disk failure in a disk array. Use a hot spare if, in RAID 5, RAID 1/0, or RAID 1 groups, high availability is so important that you want to regain data redundancy as soon as possible if a disk module fails. A hot spare provides no data storage but enhances the availability of each RAID 5, RAID 1, and RAID 1/0 group in a disk array. Disk arrays keep hot spares in use all of the time.

	NOTE: For disks managed by LVM, there is a similar feature called automatic sparing. See “Maintaining High Availability in the Event of Disk Failure” for details.

An active hot spare is differentiated from traditional hot spares in that rebuild space is distributed across all disks in the array for those disk arrays that provide active spares. This allows user data to be stored on a “spare disk,” which improves I/O performance. It also increases the amount of high performing RAID 1 space. In other words, the active hot spare disk is constantly undergoing writes and reads in order to verify that it is working properly.

In a traditional hot spare array, a defective hot spare disk may not be detected until it is actually needed. The integrity of the active hot spare is assured because it is kept in use at all times. Note that some disk arrays provide active hot spares although others do not.