Consistency protection

The Copy Services functions improve data availability and recovery if the system loses connectivity to volumes or consistency groups. When data is copied from a primary volume (master) to a secondary volume (auxiliary), consistency protection ensures that the secondary volume contains a write-order consistent set of data. The consistency of the secondary volume can be protected during resynchronization.

The Metro Mirror, Global Mirror, Global Mirror with change volumes, and HyperSwap Copy Services functions create remote copy or remote replication relationships between volumes or consistency groups. If the secondary volume in a Copy Services relationship becomes unavailable to the primary volume, the system maintains the relationship. However, the data might become out of sync when the secondary volume becomes available.

Change volumes can be used to maintain a consistent image of the secondary volume. Active-active (HyperSwap) relationships and Global Mirror relationships with cycling mode set to Multiple must always be configured with change volumes.Metro Mirror and Global Mirror with cycling mode set to None can optionally be configured with change volumes.

When a secondary change volume is configured, the relationship between the primary and secondary volumes does not stop if the link goes down or the secondary volume is offline. The relationship does not go in to the Consistent stopped status. Instead, the system uses the secondary change volume to automatically copy the previous consistent state of the secondary volume. The relationship automatically moves to the Consistent copying status as the system resynchronizes and protects the consistency of the data. The relationship status changes to Consistent synchronized when the resynchronization process completes. The relationship automatically resumes replication after the temporary loss of connectivity.

You are not required to configure a secondary change volume on a Metro Mirror or Global Mirror (without cycling) relationship. However, if the link goes down or the secondary volume is offline, the relationship goes in to the Consistent stopped status. If write operations take place on either the primary or secondary volume, the data is no longer synchronized (Out of sync).

Consistency protection must be enabled on all relationships in a consistency group. Every relationship in a consistency group must be configured with a secondary change volume. If a secondary change volume is not configured on one relationship, the entire consistency group stops with a 1720 error if host I/O is processed when the link is down or any secondary volume in the consistency group is offline. All relationships in the consistency group are unable to retain a consistent copy during resynchronization.

The option to add consistency protection is selected by default when you create Metro Mirror or Global Mirror relationships. The option must be cleared to create Metro Mirror or Global Mirror relationships without consistency protection.

To restart a relationship or consistency group that is in the Consistent stopped state, you must specify the –force option on the startrcrelationship or startrcconsistgrp command. The copy operation might lead to a temporary loss of consistency while synchronization occurs. The consistent data on the secondary volume might be lost.