How mirrored protection works

Because mirrored protection is configured by disk pool, you can mirror one, some, or all disk pools on the system.

By default, every system has a system disk pool. It is not necessary to create user disk pools in order to use mirrored protection. Although mirrored protection is configured by disk pool, all disk pools must be mirrored to provide for maximum system availability. If a disk unit fails in a disk pool that is not mirrored, the system cannot be used until the disk unit is repaired or replaced.

The start mirrored pairing algorithm automatically selects a mirrored configuration that provides the maximum protection at the bus, IOP, or IOA for the hardware configuration of the system. When disk units of a mirrored pair are on separate buses, they have maximum independence or protection. Because they do not share any resource at the bus, IOP , or IOA levels, a failure in one of these hardware components allows the other mirrored disk unit to continue operating.

Any data that is written to a disk unit that is mirrored is written to both disk units of the mirrored pair. When data is read from a disk unit that is mirrored, the read operation can be from either disk unit of the mirrored pair. It is transparent to the user which mirrored disk unit the data is being read from. A user is not aware of the existence of two physical copies of the data.

If one disk unit of a mirrored pair fails, the system suspends mirrored protection to the failed mirrored disk unit. The system continues to operate using the remaining mirrored disk unit. The failing mirrored disk unit can be physically repaired or replaced. When a hot spare disk unit is configured to protect the mirrored unit, the hot spare disk unit automatically replaces the failed mirrored disk unit after the failed mirrored disk unit has been suspended for five minutes. The five minute waiting period avoids consuming the hot spare disk unit if the mirror suspension is a temporary condition.

After the failed mirrored disk unit is repaired or replaced, the system synchronizes the mirrored pair by copying current data from the disk unit that has remained operational to the other disk unit. During synchronization, the mirrored disk unit to which the information is being copied is in the resuming state. Synchronization does not require a dedicated system and runs concurrently with other jobs on the system. System performance is affected during synchronization. When synchronization is complete, the mirrored disk unit becomes active.