530 Implementing the IBM Storwize V7000 with IBM Spectrum Virtualize V8.2.1
You can adjust the cycling period by using the chrcrelationship -cycleperiodseconds 
<60 - 86400> command from the CLI. The default value is 300 seconds. If a copy does not 
complete in the cycle period, the next cycle does not start until the prior cycle completes. For 
this reason, the use of Change Volumes gives you the following possibilities for RPO: 
/SM590000If your replication completes in the cycling period, your RPO is twice the cycling period.
/SM590000If your replication does not complete within the cycling period, RPO is twice the completion 
time. The next cycling period starts immediately after the prior cycling period is finished.
Carefully consider your business requirements versus the performance of Global Mirror with 
Change Volumes. Global Mirror with Change Volumes increases the intercluster traffic for 
more frequent cycling periods. Therefore, selecting the shortest cycle periods possible is not 
always the answer. In most cases, the default must meet requirements and perform well.
11.6.14  Distribution of work among nodes
For the best performance, MM/GM volumes must have their preferred nodes evenly 
distributed among the nodes of the systems. Each volume within an I/O Group has a 
preferred node property that can be used to balance the I/O load between nodes in that 
group. MM/GM also uses this property to route I/O between systems.
If this preferred practice is not maintained, such as if source volumes are assigned to only 
one node in the I/O group, you can change the preferred node for each volume to distribute 
volumes evenly between the nodes. You can also change the preferred node for volumes that 
are in a remote copy relationship without affecting the host I/O to a particular volume. 
The remote copy relationship type does not matter. The remote copy relationship type can be 
Metro Mirror, Global Mirror, or Global Mirror with Change Volumes. You can change the 
preferred node both to the source and target volumes that are participating in the remote copy 
relationship.
11.6.15  Background copy performance
The background copy performance is subject to sufficient RAID controller bandwidth. 
Performance is also subject to other potential bottlenecks, such as the intercluster fabric, and 
possible contention from host I/O for the IBM Spectrum Virtualize system bandwidth 
resources.
Background copy I/O is scheduled to avoid bursts of activity that could have an adverse effect 
on system behavior. An entire grain of tracks on one volume is processed at around the same 
time, but not as a single I/O. Double buffering is used to try to use sequential performance 
within a grain. However, the next grain within the volume might not be scheduled for some 
time. Multiple grains might be copied simultaneously, and might be enough to satisfy the 
requested rate, unless the available resources cannot sustain the requested rate.
Global Mirror paces the rate at which background copy is performed by the appropriate 
relationships. Background copy occurs on relationships that are in the InconsistentCopying 
state with a status of Online.
Important: When you create your Global Mirror Volumes with Change Volumes, make 
sure that you remember to select the change volume on the auxiliary (target) site. Failure 
to do so leaves you exposed during a resynchronization operation.