Managing Systems and Workgroups: A Guide for HP-UX System Administrators
Using High Availability Strategies
Using Hot Spared Disks
Appendix A 955
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” on page 640 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.