Device parity protection is a hardware availability function that protects data from being lost because of disk unit failure or because of damage to a disk. To protect data, the disk input/output adapter (IOA) calculates and saves a parity value for each bit of data. Conceptually, the IOA computes the parity value from the data at the same location on each of the other disk units in the device parity set. When a disk failure occurs, the data can be reconstructed by using the parity value and the values of the bits in the same locations on the other disks. The system continues to run while the data is being reconstructed. The overall goal of device parity protection is to provide high availability and to protect data as inexpensively as possible.
Two types of device parity protection
RAID 5 and RAID 6 are the two types of device parity protection.
RAID 5
If more than one disk fails, you must restore the data from the backup media. Logically, the capacity of one disk unit is dedicated to storing parity data in a parity set. However, in practice the parity data is spread among multiple disk units. Restoring data to a disk pool that has disk units with device parity protection may take longer than a disk pool that contains only unprotected disk units.
| Number of disk units in a parity set | Number of disk units that store parity |
|---|---|
| 3 | 2 |
| 4-7 | 4 |
| 8-15 | 8 |
| 16-18 | 16 |
RAID 6
If more than two disk units fail, you must restore the data from the backup media. Logically, the capacity of two disk units is dedicated to storing parity data in a parity set. However, in practice the parity data is spread among multiple disk units.
The minimum number of disk units in a parity set is 4. The maximum number of disk units in a parity set is 18.
When a RAID 6 parity set is started, all of the disk units contain parity. Restoring data to a disk pool that has disk units with device parity protection may take longer than a disk pool that contains only unprotected disk units.
Raid 6 requires a new storage adapter that supports this new
function. The 571B is the first adapter to support Raid 6.
Device parity protection is not a substitute for a backup and recovery strategy
Device parity protection is not a substitute for a backup and recovery strategy. Device parity protection can prevent your system from stopping when certain types of failures occur. It can speed up your recovery process for certain types of failures. But device parity protection does not protect you from many types of failures, such as a site disaster or an operator or programmer error. It does not protect against system outages that are caused by failures in other disk-related hardware (such as IOAs, disk I/O processors, or a system bus).
If possible, you should protect all the disk units on your system with either device parity protection or Work with mirrored protection. This prevents the loss of information when disk failure occurs. In many cases, you can also keep your system operational while a disk unit is being repaired or replaced.
For information about
how to start using device parity protection, see Backup
and Recovery
.