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.