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 SAN 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 SAN cannot sustain the workload, the Global Mirror
relationships are automatically stopped to protect your application hosts from increased response
times.
The system supports the following types of relationships and consistency
groups:
- Active-active (for HyperSwap®
volumes)
- Metro
Mirror
- Global Mirror without
cycling (cycling mode set to None)
- Global Mirror with change volumes
(cycling mode set to Multiple)
Change volumes can be used in a number of cases with relationships. Active-active
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, which can be used to
maintain a consistent secondary image.
If necessary, you can change the copy type of a Metro
Mirror or Global Mirror remote-copy relationship or
consistency group without re-creating the relationship or consistency group with the different type.
For example, if the latency of the long-distance link affects host performance, you can change the
copy type to Global Mirror to improve
host performance over high latency links. For Global Mirror relationships with multiple
cycling mode, changes are tracked and copied to intermediate change volumes. The changes are
transmitted to the secondary site periodically to lower bandwidth requirements. 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.Note: You cannot change the type of a HyperSwap
(active-active) relationship or consistency group.
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.