## Background copy bandwidth effect on foreground I/O latency 

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. 

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. 

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. 

## The chpartnership command 

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. 

## 10.9.5 Creating a Metro Mirror/Global Mirror consistency group 

Use the mkrcconsistgrp  command to create an empty MM/GM Consistency Group. 

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. 

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. 

## 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. 

