Metro Mirror and Global Mirror relationships

Metro Mirror and Global Mirror relationships define the relationship between two virtual disks (VDisks): a master VDisk and an auxiliary VDisk.

Typically, the master VDisk contains the production copy of the data and is the VDisk that the application normally accesses. The auxiliary VDisk typically contains a backup copy of the data and is used for disaster recovery.

The master and auxiliary VDisks are defined when the relationship is created, and these attributes never change. However, either VDisk can operate in the primary or secondary role as necessary. The primary VDisk contains a valid copy of the application data and receives updates from the host application, analogous to a source VDisk. The secondary VDisk receives a copy of any updates to the primary VDisk, because these updates are all transmitted across the Mirror link. Therefore, the secondary VDisk is analogous to a continuously updated target VDisk. When a relationship is created, the master VDisk is assigned the role of primary VDisk and the auxiliary VDisk is assigned the role of secondary VDisk. Therefore, the initial copying direction is from master to auxiliary. When the relationship is in a consistent state, you can reverse the copy direction from the command-line interface (CLI) or the SAN Volume Controller Console.

The two VDisks in a relationship must be the same size. When the two VDisks are in the same cluster, they must be in the same I/O group.

A relationship can be added to a consistency group, for ease of application management.

Note: Membership of a consistency group is an attribute of the relationship, not the consistency group. Therefore, issue the svctask chrcrelationship command to add or remove a relationship to or from a consistency group.

Copy types

A Metro Mirror copy ensures that updates are committed to both the primary and secondary VDisks before sending confirmation of I/O completion to the host application. This ensures that the secondary VDisk is synchronized with the primary VDisk in the event that a failover operation is performed.

A Global Mirror copy allows the host application to receive confirmation of I/O completion before the updates are committed to the secondary VDisk. If a failover operation is performed, the host application must recover and apply any updates that were not committed to the secondary VDisk.

States

When a Metro Mirror or Global Mirror relationship is created with two VDisks in different clusters, the distinction between the connected and disconnected states is important. These states apply to both clusters, the relationships, and the consistency groups. The following Metro Mirror and Global Mirror relationship states are possible:

InconsistentStopped: The primary VDisk is accessible for read and write I/O operations, but the secondary VDisk is not accessible for either operation. A copy process must be started to make the secondary VDisk consistent.
InconsistentCopying: The primary VDisk is accessible for read and write I/O operations, but the secondary VDisk is not accessible for either operation. This state is entered after an svctask startrcrelationship command is issued to a consistency group in the InconsistentStopped state. This state is also entered when an svctask startrcrelationship command is issued, with the force option, to a consistency group in the Idling or ConsistentStopped state.
ConsistentStopped: The secondary VDisk contains a consistent image, but it might be out of date with respect to the primary VDisk. This state can occur when a relationship was in the ConsistentSynchronized state and experiences an error that forces a freeze of the consistency group. This state can also occur when a relationship is created with the CreateConsistentFlag parameter set to TRUE.
ConsistentSynchronized: The primary VDisk is accessible for read and write I/O operations. The secondary VDisk is accessible for read-only I/O operations.
Idling: A master VDisk and an auxiliary VDisk operates in the primary role. Consequently the VDisk is accessible for write I/O operations.
IdlingDisconnected: The VDisks in this half of the consistency group are all operating in the primary role and can accept read or write I/O operations.
InconsistentDisconnected: The VDisks in this half of the consistency group are all operating in the secondary role and cannot accept read or write I/O operations.
ConsistentDisconnected: The VDisks in this half of the consistency group are all operating in the secondary role and can accept read I/O operations but not write I/O operations.