Published: November 12, 2007   |   Updated: February 25, 2008

The purpose of step 9 is to determine which specific applications will be placed on which specific physical hosts. The mapping process involves using information collected in previous steps. For example, if several applications are unable to coexist on a server based on physical security or coexistence issues, they must be mapped onto different host servers.

Task 1: Determine Host Resource Limits

The overall process of determining the ideal mapping of guest computers to specific hosts can require significant effort. The process of taking into account the many different considerations for resources and other requirements can lead to many viable options worth considering. Additionally, workload characteristics can change over time. Therefore, consider details such as target host resource use.

Determine the Threshold/Buffer Plan

From the standpoint of host servers, the organization must decide on the optimum host resource use levels. One approach is to maximize resource use on each host while leaving sufficient “head room” for handling unexpected peaks in use patterns. For example, it might be determined that the optimum CPU utilization for virtualization host servers is 80 percent.

Task 2: Map Guests to the Form Factor

This task involves allocating the guest-related workload requirements (summarized in step 6) to the physical host machines.

Use information about specific VM resource requirements to determine how best to place VMs. For example, categorize workloads based on their CPU requirements. In addition to resource requirements, consider combining or separating those applications based on backup, availability, and isolation requirements. For example, if entire VM backup is required, deploy the application onto servers designated for this method.

Ideally, this process would be likened to an automatic coin counter where all of the coins are put in a funnel in the top of the machine and the coins are automatically separated into their respective denominations and stacked in sizes that fit the coin wrappers. When a stack is full, the machine stops until the full wrapper is removed and a new one is positioned.

Unfortunately, most of this process is manual today so it is more analogous to a child’s shape sorting toy where each object must be manually placed into the right location.

The goal of this part of the design is to map all the applications onto as few physical servers as possible in each geography while still respecting the needs for server buffer, isolation, availability, backup type, and so on.

At the end of this exercise, all applications and requirements should be accounted for and the number of physical servers should be identified. Additional servers may be required to meet fault tolerance requirements. These are addressed later in this guide. It may be appropriate to iterate through this process using different form factor designs to find the optimal configuration.

Validating with the Business

IT can complete resource-related requirements for mapping guests to the host infrastructure. However, other details, such as fault tolerance and backup requirements, should be verified with the business. Considerations include:

• Are these decisions consistent with the organization’s goals for virtualization? Determine the method of workload allocation based on the expected business benefits of adding a virtual infrastructure.
• Does the current design meet the organization’s availability and backup requirements? Details such as host server placement, facility limitations, and systems administrator expertise can affect overall operations.
• Which requirements and constraints can be considered flexible? In some cases, businesses might find that relatively small investments in additional hardware can lead to better overall workload distribution decisions. The organization should reconsider details such as budgets and employee resources based on results from this step.

Decision Summary

Determining how best to distribute guest workloads onto a virtual infrastructure can take significant time and effort. The specific considerations vary widely based on the number and type of applications to be supported in addition to preliminary decisions based on the host hardware infrastructure. Plan to regularly review these decisions after making changes to either the guest or host requirements.