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 HP-UX
System Administrator’s Guide: Logical Volume Management 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.