Changing the copy type of remote-copy relationships and consistency groups

You can change the copy type of both relationships and consistency groups between Metro Mirror and Global Mirror.

The system supports the following types of relationships:

Active-active

This type of relationship is only created for HyperSwap® volumes. When HyperSwap is configured on the system, the HyperSwap volumes are located on separate sites and an active-active relationship is automatically configured between them. Updates to the volumes in the relationship are updated simultaneously on both sites to provide disaster recovery solutions for the system.

Metro Mirror

Metro Mirror is a type of remote copy that creates a synchronous copy of data from a primary volume to a secondary volume. A secondary volume can either be on the same system or on another system.

With synchronous copies, host applications write to the primary volume but do not receive confirmation that the write operation has completed until the data is written to the secondary volume. This ensures that both the volumes have identical data when the copy operation completes. After the initial copy operation completes, the Metro Mirror function maintains a fully synchronized copy of the source data at the target site at all times.

The Metro Mirror function supports copy operations between volumes that are separated by distances up to 300 km. For disaster recovery purposes, Metro Mirror provides the simplest way to maintain an identical copy on both the primary and secondary volumes. However, like with all synchronous copies over remote distances, there can be a performance impact to host applications. This performance impact is related to the distance between primary and secondary volumes and depending on application requirements, its use might be limited based on the distance between sites.

Global Mirror without cycling (cycling mode set to None)

The Global Mirror function provides an asynchronous copy process. When a host writes to the primary volume, confirmation of I/O completion is received before the write operation completes for the copy on the secondary volume.

If a failover operation is initiated, the application must recover and apply any updates that were not committed to the secondary volume. If I/O operations on the primary volume are paused for a small length of time, the secondary volume can become an exact match of the primary volume. This function is comparable to a continuous backup process in which the last few updates are always missing. When you use Global Mirror for disaster recovery, you must consider how you want to handle these missing updates.

To use the Global Mirror function, all components in the network must be capable of sustaining the workload that is generated by application hosts and the Global Mirror background copy process. If all of the components in the network cannot sustain the workload, the Global Mirror relationships are automatically stopped to protect your application hosts from increased response times.

When Global Mirror operates without cycling, write operations are applied to the secondary volume as soon as possible after they are applied to the primary volume. The secondary volume is generally less than 1 second behind the primary volume, which minimizes the amount of data that must be recovered if a failover occurs. However, a high-bandwidth link must be provisioned between the two sites.

Global Mirror with change volumes (cycling mode set to Multiple)

Global Mirror with change volumes (cycling mode set to Multiple) provides the same basic function of asynchronous copy operations between source and target volumes for disaster recovery.

If you are using Global Mirror with cycling mode set to Multiple, the copying process is similar to Metro Mirror and standard Global Mirror. Change volumes must be configured for both the primary and secondary volumes in each relationship. A copy is taken of the primary volume in the relationship using the change volume that is specified when the Global Mirror relationship with change volumes is created. The background copy process reads data from the stable and consistent change volume, copying the data to the secondary volume in the relationship. Copy-on-write technology is used to maintain the consistent image of the primary volume for the background copy process to read. The changes that took place while the background copy process was active are also tracked. The change volume for the secondary volume can also be used to maintain a consistent image of the secondary volume while the background copy process is active.

You can change the copy type by using either the management GUI or the command-line interface. However, before you change the copy type of either a relationship or consistency group, consider the following requirements:

Any relationship or consistency group must be in the stopped state (Inconsistent Stopped, Consistent Stopped, or Idling state).
When you change the type for a consistency group, all relationships within the group also change their copy type.

Use the management GUI to change the copy type

You can use the management GUI to change the copy type of different types of relationships.

To change a single relationship, complete the following steps.
1. Select Copy Services > Remote Copy.
2. Expand Not in a group and right-click the relationship that you want to change and select Edit Relationship.
3. In the Edit Relationship dialog, select either Metro or Global for the new copy type. If you are changing the copy type from a Metro to a Global copy type, you need to specify whether to use cycling mode. Set the value to None if you do not want change volumes that are created for this relationship. Set the value to Multiple to create change volumes for this relationship. With multiple cycling mode, changes are tracked and copied to intermediate change volumes and are transferred to the secondary volume periodically to reduce bandwidth requirements. If you select Multiple for the cycling mode, you can specify the number of seconds between copy cycles for the relationship.
4. Click OK.
To update a consistency group, complete these steps:
1. Select Copy Services > Remote Copy.
2. Right-click the consistency group and select Edit Consistency Group.
3. In the Edit Consistency Group dialog, select either Metro or Global for the new copy type. If you are changing the copy type from a Metro to a Global copy type, you need to specify whether to use cycling mode. Set the value to None if you do not want change volumes that are created for this relationship. Set the value to Multiple to create change volumes for this relationship. With multiple cycling mode, changes are tracked and copied to intermediate change volumes and are transferred to the secondary volume periodically to reduce bandwidth requirements. If you select Multiple for the cycling mode, you can specify the number of seconds between copy cycles for the consistency group. The new copy type and settings apply to all the relationships within the group.
4. Click OK.
To change a relationship or a consistency group that had existing change volumes or to change to a Global Mirror with change volumes, you must change both the master and auxiliary systems. To do so, complete the following steps.
1. On the master system, select Copy Services > Remote Copy in the management GUI, right-click the relationship or consistency group that you changed and select Global Mirror Change Volumes. You can select one of the following options:
  Create New
  
  Select this option if you are changing the copy type from either a Metro Mirror or Global Mirror relationship or consistency group to a Global Mirror with change volumes.
  
  Add Existing
  
  Select this option if the system contains existing change volumes.
  
  Delete
  
  Select this option if you are changing the copy type from a Global Mirror with change volumes to either a Global Mirror without change volumes or a Metro Mirror relationship or consistency group.
2. On the auxiliary system, select Copy Services > Remote Copy in the management GUI, right-click the relationship or consistency group that you changed and select Global Mirror Change Volumes. You can select one of the following options:
  Create New
  
  Select this option if you are changing the copy type from either a Metro Mirror or Global Mirror relationship or consistency group to a Global Mirror with change volumes.
  
  Add Existing
  
  Select this option if the existing change volumes are on the system.
  
  Delete
  
  Select this option if you are changing the copy type from a Global Mirror with change volumes to either a Global Mirror without change volumes or a Metro Mirror relationship or consistency group.

Use the CLI to change the copy type

You can enter CLI commands to change the copy type of a relationship. The command and parameters that you enter vary depending on the type of a relationship.

If you are changing a Metro Mirror relationship to a Global Mirror relationship without cycling mode, enter this command, where rc_rel_name is the name of the relationship that you are changing.
```
chrcrelationship -global -cyclingmode none rc_rel_name
```
If you are changing a Metro Mirror relationship to a Global Mirror relationship with multiple cycling mode, complete these steps:
1. On either the master or auxiliary system, run the following commands. The period value specifies the number of seconds that a cycling mode is completed for the relationship and rc_rel_name is the name of the relationship that you are changing.
```
chrcrelationship -global -cyclingmode multi rc_rel_name 
```
```
chrcrelationship -cycleperiodseconds period rc_rel_name 
```
2. On the master system, run the following command, where master_change_vdisk_name is the name of the change volume that is associated with the master volume.
```
chrcrelationship -masterchange master_change_vdisk_name
```
3. On the auxiliary system, run the following command, where aux_change_vdisk_name is the name of the change volume that is associated with the auxiliary volume.
```
chrcrelationship -auxchange aux_change_vdisk_name
```
If you are changing a Global Mirror relationship to a Metro Mirror relationship, enter the following command, where rc_rel_name is the name of the relationship that you are changing.
```
chrcrelationship -metro rc_rel_name
```
To change the copy type of a consistency group, enter one of the following commands.
- If you are changing a Metro Mirror consistency group to a Global Mirror consistency group without cycling mode, enter this command, where rc_consist_group_name is the name of the consistency group that you are changing. All new settings apply to all the relationships within the group.
```
chrcconsistgrp -global -cyclingmode none rc_consist_group_name
```
- If you are changing a Metro Mirror consistency group to a Global Mirror consistency group with multiple cycling mode, enter this command. The period value is the number of seconds that a cycling mode is completed for the relationship and rc_consist_group_name is the name of the consistency group that you are changing. All new settings apply to all the relationships within the group.
```
chrcconsistgrp -global -cyclingmode multi -cycleperiodseconds period rc_consist_group_name
```
- If you are changing a Global Mirror consistency group to a Metro Mirror consistency group, enter this command, where rc_consist_group_name is the name of the consistency group that you are changing. All new settings apply to all the relationships within the group.
```
chrcconsistgrp  -metro rc_consist_group_name
```