There are two main factors that determine whether or not you need Secondary Servers in your system:
Number of devices: The number of devices you intend to manage is one of the major factors in determining the number of ZENworks Servers you need. How many management tasks you'll be regularly performing on these devices is another contributing factor.
Network speed: The ZENworks system has three key communication paths that should not be impeded by slow WAN links: 1) the ZENworks Primary Server to a ZENworks Secondary Server, 2) any ZENworks Servers to the ZENworks Data Store, and 3) any managed device to its ZENworks Server.
In general, this means that to achieve the best performance, you should do the following:
Make sure that each managed device resides in the same local area network as its ZENworks Server. For example, if you have devices in a Los Angeles office and devices in a London offices and the two offices have a slow WAN link, you should have ZENworks Servers in both offices.
Make sure that all Secondary Servers reside in the same local area network as the Primary Server. The Primary Server replicates software packages and images to each Secondary Server in its Management Zone on a regularly scheduled basis. If there is a large amount of data to transfer, this process can consume extensive bandwidth.
Make sure that ZENworks Servers reside in the same local area network as the ZENworks Data Store. The servers consistently access the Data Store for ZENworks information.
Overall, you achieve best performance if all ZENworks Servers and managed devices within the same Management Zone are located on the same LAN or have fast WAN links. To facilitate consistency between different Management Zones, you can mirror the software packages between Primary Servers in the different zones (see Novell ZENworks 7.3 Linux Management Administration Guide).