Global Mirror or Metro Mirror change volumes create periodic point-in-time copies of
either the primary volumes or the secondary volumes. You can use the command-line interface to
create, change, or remove change volumes.
Change volumes can be used in a number of cases with relationships:
- Active-active relationships and relationships with
cycling modes 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, which can be used to
maintain a consistent secondary image when a relationship is resynchronized following an
interruption to the replication.
For
Global Mirror relationships
with multiple cycling modes, changes are tracked and copied to intermediate change volumes at the
primary site. The changes are transmitted to the secondary site continually to lower the bandwidth
requirements, requiring only the average throughput not the peak. If you are changing either a
Metro Mirror or
Global Mirror relationship or consistency
group to a Global Mirror with change volumes (cycling mode set to
Multiple),
you must create change volumes on both the master and auxiliary volumes that are used in the
relationship or consistency group.
Using the command-line interface
- If the master and auxiliary volumes do not exist, use the mkvdisk command to
create the change volumes. Master change volumes must be created with the following restrictions:
- Master change volumes must be in the same I/O group.
- Master change volumes must be the same logical size as the master volume of the
relationship.
- Master change volumes must be in the same storage pool as the master volume.
This process is the same for auxiliary change volumes. When you assign a change volume to a
relationship, the system creates new FlashCopy mappings between the master or auxiliary volume and
the associated change volume. Therefore, sufficient available FlashCopy memory is needed in the
target I/O group or the command fails.
- To create the change volume relationship on the master and auxiliary volumes, use the
chrcrelationship command:
chrcrelationship -masterchange master_change_vdisk_id | master_change_vdisk_name
chrcrelatinship -auxchange aux_change_vdisk_id | aux_change_vdisk_name
- To remove change volumes, also use the chrcrelationship
command:
chrcrelationship -nomasterchange
chrcrelationship -noauxchange
The -masterchange and -nomasterchange parameters can be
specified only when you run the chrcrelationship command on the master system for
the relationship. The -auxchange and -noauxchange
parameters can be specified only when you run the chrcrelationship command on the
auxiliary system for the relationship.