Concurrent maintenance is the process of repairing or replacing a failed disk-related hardware component while the system is being used for normal operations.
On systems without mirrored protection or device parity protection, the system is not available when a disk-related hardware failure occurs and remains unavailable until the failed hardware is repaired or replaced. However, with mirrored protection the failing hardware can often be repaired or replaced while the system is being used.
Concurrent maintenance support is a function of system unit hardware packaging. Mirrored protection only provides concurrent maintenance when the hardware and packaging of the system support it. The best hardware configuration for mirrored protection also provides for the maximum amount of concurrent maintenance.
It is possible for the system to operate successfully through many failures and repair actions. For example, a failure of a disk head assembly does not prevent the system from operating. A replacement of the head assembly and synchronization of the mirrored unit can occur while the system continues to run. The greater your level of protection, the more often concurrent maintenance can be performed.
On some models, the system restricts the level of protection for unit 1
and its mirrored unit to only IOA-level protection. See "Mirrored Protection
- Configuration Rules" in Backup
and Recovery 
for more information.
Under some conditions, diagnosis and repair can require active mirrored units to be suspended. You may prefer to turn off the system to minimize the exposure of operating with less mirrored protection. Some repair actions require that the system be powered down. Deferred maintenance is the process of waiting to repair or replace a failed disk-related hardware component until the system can be powered down. The system is available, although mirrored protection is reduced by whatever hardware components have failed. Deferred maintenance is only possible with mirrored protection or device parity protection.