Background copy considerations The remote copy partnership bandwidth parameter  explicitly  defines the rate at which the  background copy is attempted, but also  implicitly  affects foreground I/O. Background copy  bandwidth can affect foreground I/O latency in one of the following ways: If the remote copy partnership bandwidth parameter is set too high for the actual  intersystem network capability, the background copy resynchronization writes use too  much of the intersystem network. It starves the link of the ability to service synchronous or  asynchronous mirrored foreground writes. Delays in processing the mirrored foreground  writes increase the latency of the foreground I/O as perceived by the applications.  If the remote copy partnership background copy rate is set too high, the added read I/Os  that are associated with background copy writes can overload the storage at the primary  site and delay foreground (read and write) I/Os.  If the remote copy partnership background copy rate is set too high for the storage at the  secondary site, the background copy writes overload the auxiliary storage. Again, they  delay the synchronous and asynchronous mirrored foreground write I/Os. Important:  An increase in the peak foreground workload can have a detrimental effect  on foreground I/O. It does so by pushing more mirrored foreground write traffic along  the intersystem network, which might not have the bandwidth to sustain it. It can also  overload the primary storage. To set the background copy bandwidth optimally, consider all aspects of your environments,  starting with the following biggest contributing resources: 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. In cases where the  available network bandwidth is not able to sustain an acceptable background copy rate,  consider alternatives to the initial copy as described in “Initial synchronization options and  Offline Synchronization” on page 232. Changes in the environment, or loading of it, can affect the foreground I/O. IBM Spectrum  Virtualize and Storwize technology provides a means to monitor, and a parameter to control,  how foreground I/O is affected by running remote copy processes. IBM Spectrum Virtualize  software monitors the delivery of the mirrored foreground writes. If latency or performance of  these writes extends beyond a (predefined or client-defined) limit for a period, the remote  copy relationship is suspended (see 5.3.5, “1920 error” on page 240). Finally, note that with Global Mirror Change Volume, the cycling process that transfers the  data from the local to the remote system is a background copy task. For this reason, the  background copy rate, as well as the  relationship_bandwidth_limit , setting affects the  available bandwidth not only during the initial synchronization, but also during the normal  cycling process. Chapter 5. Copy services  231 GLYPH<SM590000> Primary storage GLYPH<SM590000> Intersystem network bandwidth  GLYPH<SM590000> Auxiliary storage  GLYPH<SM590000> Increasing latency of foreground I/O GLYPH<SM590000> Read I/O overload of primary storage GLYPH<SM590000> Write I/O overload of auxiliary storage