We're just starting HPC planning. We also need a Linux cluster, so started there, and see lots of talk of infiniband. But would that be needed for the Windows HPC cluster? In Windows, we're looking at one head node, 4 compute nodes. If I understand this
correctly, the 4 compute nodes will eventually need to write results to a shared datastore, which I think would be local disk on the head node.
No, Infiniband is an optional part of the deployment. If you want to have really fast network you can buy it, but it is not so cheap (especially Infiniband QDR and so on). And even when you deploy Infiniband, you can utilize your Ethernet network as backup
We have a Windows Cluster based on HPC Server 2008 withour Infiniband installed (but it is present on network) and all is work like a charm.