Chapter 3. Planning 77
Table 3-5 lists the amount of heartbeat traffic (in megabits per second) that is generated by 
various sizes of clustered systems.
Table 3-5   Intersystem heartbeat traffic in Mbps
These numbers estimate the amount of traffic between the two clustered systems when no 
I/O is taking place to mirrored volumes. Half of the data is sent by each of the systems. The 
traffic is divided evenly over all available intercluster links. Therefore, if you have two 
redundant links, half of this traffic is sent over each link.
The bandwidth between sites must be sized to meet the peak workload requirements. You 
can estimate the peak workload requirement by measuring the maximum write workload 
averaged over a period of 1 minute or less, and adding the heartbeat bandwidth. Statistics 
must be gathered over a typical application I/O workload cycle, which might be days, weeks, 
or months, depending on the environment in which the Storwize V7000 is used.
When planning the inter-site link, consider also the initial sync and any future resync 
workloads. It might be worthwhile to secure additional link bandwidth for the initial data 
synchronization.
If the link between the sites is configured with redundancy so that it can tolerate single 
failures, you must size the link so that the bandwidth and latency requirements are met even 
during single failure conditions.
When planning the inter-site link, make a careful note whether it is dedicated to the 
inter-cluster traffic or is going to be used to carry any other data. Sharing link with other traffic 
(for example, cross-site IP traffic) might reduce the cost of creating the inter-site connection 
and improve link utilization. However, doing so might affect the links’ ability to provide the 
required bandwidth for data replication.
Verify carefully that the devices that you plan to use to implement the intercluster link are 
supported. 
Cluster configuration
If you configure replication services, you might decide to dedicate ports for intercluster 
communication, intracluster traffic, or both. In that case, make sure that your cabling and 
zoning reflects that decision. Also, such dedicated ports are inaccessible for host or back-end 
storage traffic, so plan your volume mappings and hosts and back-end storage connections 
accordingly.
Storwize V7000 
System 1
Storwize V7000 System 2
2 nodes 4 nodes 6 nodes 8 nodes
2 nodes 5 06 06 06
4 nodes 61 0 1 1 1 2
6 nodes 61 1 1 6 1 7
8 nodes 61 2 1 7 2 1