Background copy bandwidth effect on foreground I/O latency 10.9.5 Creating a Metro Mirror/Global Mirror consistency group The chpartnership command 10.9.6 Creating a Metro Mirror/Global Mirror relationship Use the  mkrcrelationship  command to create a MM/GM relationship. This relationship  persists until it is deleted. Optional parameter:  If you do not use the  -global  optional parameter, a Metro Mirror  relationship is created rather than a Global Mirror relationship. The new consistency group does not contain any relationships and is in the  Empty  state. You  can add MM/GM relationships to the group (upon creation or afterward) by using the  chrcrelationship  command. The MM/GM consistency group name must be unique. If the consistency group involves two  systems, the systems must be in communication throughout the creation process. Use the  mkrcconsistgrp  command to create an empty MM/GM Consistency Group. To change the bandwidth that is available for background copy in the system partnership, use  the  chpartnership -backgroundcopyrate < percentage_of_link_bandwidth>  command to  specify the percentage of whole link capacity to be used by background copy process. Perform this provisioning by calculation or by determining experimentally how much  background copy can be allowed before the foreground I/O latency becomes unacceptable.  Then, reduce the background copy to accommodate peaks in workload.  To set the background copy bandwidth optimally, ensure that you consider all three resources:  primary storage, intercluster link bandwidth, and auxiliary storage. Provision the most  restrictive of these three resources between the background copy bandwidth and the peak  foreground I/O workload.  The background copy bandwidth determines the rate at which the background copy is  attempted for MM/GM. The background copy bandwidth can affect foreground I/O latency in  one of the following ways: GLYPH<SM590000> The following result can occur if the background copy bandwidth is set too high compared  to the MM/GM intercluster link capacity:  – The background copy I/Os can back up on the MM/GM intercluster link. – There is a delay in t he synchronous auxiliary writes of foreground I/Os. – The foreground I/O latency increases as perceived by applications. GLYPH<SM590000> If the background copy bandwidth is set too high for the storage at the primary site,  background copy read I/Os overload the primary storage and delay foreground I/Os.  GLYPH<SM590000> If the background copy bandwidth is set too high for the storage at the secondary site,  background copy writes at the secondary site overload the auxiliary storage and again  delay the synchronous secondary writes of foreground I/Os. Chapter 10. Copy Services  563