Chapter 10. Copy services 551
Global Mirror (GM) and GM with Change Volumes
Referred to as
asynchronous replication, GM provides a consistent copy of a source
virtual disk on a target virtual disk. Data is written to the target virtual disk asynchronously
so that the copy is continuously updated. However, the copy might not contain the last few
updates if a disaster recovery (DR) operation is performed. An added extension to GM is
GM with Change Volumes. GM with Change Volumes is the preferred method for use with
native IP replication.
10.6.2 IBM Storwize System Layers
An IBM Storwize family system can be in one of the two layers: the replication layer or the
storage layer. The system layer affects how the system interacts with IBM Storwize V5000
Gen2 systems and IBM SAN Volume Controller systems. The IBM SAN Volume Controller is
always set to replication layer and this parameter is unchangeable.
In the storage layer, an Storwize family system has the following characteristics and
requirements:
The system can perform MM and GM replication with other storage-layer systems.
The system can provide external storage for replication-layer systems or IBM SAN Volume
Controller.
The system cannot use a storage-layer system as external storage.
In the replication layer, an IBM SAN Volume Controller or an IBM Storwize family system has
the following characteristics and requirements:
The system can perform MM and GM replication with other replication-layer systems or
IBM SAN Volume Controller.
The system cannot provide external storage for a replication-layer system or an IBM SAN
Volume Controller.
The system can use a storage-layer system as external storage.
A Storwize family system is in the storage layer by default, but the layer can be changed. For
example, you might want to change a Storwize V5000 to a replication layer to complete
Global Mirror or Metro Mirror replication with a SAN Volume Controller system.
To change the storage layer of an existing Storwize system with internal storage before you
add a second system into the SAN zone, you do not need to stop I/O operations. However, if
the system has Fibre Channel (FC) connections to another Storwize family or SAN Volume
Controller system in the SAN fabric, I/O operations must be stopped temporarily. In this
scenario, the FC ports must be disabled (for example, by unplugging all the FC ports,
changing zoning, disabling switch ports) before you change the system layer. Then, you must
re-enable the FC ports.
Note: Before you change the layer of a Storwize family system, the following conditions
must be met:
No host object can be configured with worldwide port names (WWPNs) from a Storwize
family system.
No system partnerships can be defined.
No Storwize family system can be visible on the SAN fabric.
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