What do we mean by horizontal and vertical growth? Horizontal growth is based on the concept of adding more of the same building blocks in our case, adding server images alongside the existing server images. If you lined these servers up in a row, they would extend out to the horizon, as shown in Figure 4-1.
Figure 4-1. Horizontal growth
Such companies usually have a very segmented business where it is simple to segregate and grow by a cloning-like process. ISPCompany (for its outsourcing side of the business) experiences most of its growth from adding more images. For security reasons, each ISPCompany client needs a separate set of images. For ISPCompany's Internet service provider business, it is simple to grow by just adding more images for mail serving or Web serving as more clients sign up.
Vertical growth is based on the concept of an application needing more and more capacity. (See Figure 4-2.) In this case, it is easier to grow the image size than it is to split the work among a set of images. You could visualize this as an image growing upward:
Figure 4-2. Vertical growth
Both the horizontal and vertical growth scenarios can map well to zSeries: