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How Virtual Machine Manager Rates Hosts

When you deploy a virtual machine, Virtual Machine Manager (VMM) evaluates the suitability of all available hosts and assigns each host a rating of 0 (not suitable) through 5 stars (very suitable). The host rating is a recommendation based on the virtual machine's hardware and resource requirements and the anticipated resource usage. The administrator can override these recommendations or customize the way they are generated.

Using Host Ratings to Select Hosts

By using host ratings, you can establish customized criteria for placing virtual machines on the most suitable host. You can specify placement goals that will be used by the host ratings to select a host. The placement goals are described in the following table:

Goal Description Strategy

Resource maximization

Consolidates multiple low-utilization workloads on a single host. Virtual machine are placed on that host until the limits are reached.

Use this option if you want to fully utilize one host before assigning virtual machines to additional hosts.

Load balancing

Minimizes the processing load on any one host.

Use this option if you want to distribute the performance load of your virtual machines evenly across your hosts.

For more information, see the "How to Set Placement Defaults for Virtual Machines" Help topic (

You can select which resources to use and designate the relative importance of each resource when rating the virtual server hosts. The available resources are:

  • CPU utilization
  • Memory utilization
  • Disk I/O
  • Network utilization

For example, if you are creating a Windows Server 2003-based virtual machine for developers and know that you require significant CPU, but you have relatively little disk or network resources, you might customize the ratings to increase the priority of free CPU while lowering the priority of the disk and network.

Automatic Placement

If you use the drag-and-drop method to migrate a virtual machine to a host group in Virtual Machines view, the virtual machines are automatically placed on the most suitable host. This process is known as automatic placement.

During automatic placement, the configuration files for the virtual machine are moved to the most suitable volume on the selected host. For automatic placement to succeed, you must configure a virtual machine path on the recommended volume. For more information, see the "About Default Virtual Machine Paths" Help topic (

VMM includes the Virtual Server path in the default paths on each host as part of the automatic placement process. This ability allows you to use storage area networks (SANs) with multiple volumes. For more information, see “Moving from Virtual Server to Virtual Machine Manager” in the Virtual Machine Manager Operations Guide (

All virtual machines for self-service users are deployed through automatic placement. If you do not configure a virtual machine path on the recommended volume, virtual machine placement will fail. For more information about adding a virtual machine path to the host, see the “How to Set Placement Options for a Host” Help topic (

How Hosts are Rated

VMM evaluates all hosts within the selected host group and also any hosts contained in child host groups. The ratings are based on the placement options and do not change based on the ratings of other hosts.

Each host must be able to accommodate the host reserves and the maximum resource allocation for all of the hosted virtual machines. Host reserves are the resources set aside on the physical host computer for the host operating system.

Measuring Resource Usage

Virtual Machine Manager measures CPU usage, disk I/O, and network usage for each host approximately every 10 minutes and recalculates the average rating. The reported usage is an average of all the measurements taken since the last event that changed its resource usage. Events that reset data on a host include:

  • New Virtual Machine
  • Deploy Virtual Machine
  • Store Virtual Machine
  • Migrate Virtual Machine
  • Delete Virtual Machine
  • Virtual Machine Turned On
  • Virtual Machine Turned Off, Stopped, Paused, Saved State

Calculating Host Ratings

VMM calculates host ratings on a scale from 0 through 5 stars, including half stars. Ratings are based on individual hosts and not on the relative suitability of all available hosts. VMM calculates host ratings according to specific formulas, described in the following table:

Rating Formula


[ 1 – ( CPU Utilization / (100 – CPU Reserve)) ] x CPU Weight


[ 1 – ( Memory Used / (Total Memory – Memory Reserve)) ] x Memory Weight


[ 1 – ( Disk IOPS / Maximum Disk IOPS ] x Disk Weight


[ 1 – ( Network Utilization / (100 – Network Reserve)) ] x Network Weight

Conditions That Cause a Zero Rating

If a host fails to meet one or more specific requirements, it will receive a host rating of 0 stars. Any available host which meets these requirements will automatically receive a rating of greater than 0 stars. The requirements are described in the following table:

Requirement Description

Physical disk space

The host must have at least one disk with enough space to hold the total disk space required by the virtual machine. With dynamic disks, the current disk size is used, not the maximum disk size.


The memory required for the virtual machine must be less than the current amount of memory available on the host. A host must have sufficient free memory to start the virtual machine.

Your host requires slightly more memory to start a virtual machine than it requires to place a virtual machine.

Network connectivity

The virtual networks required by the virtual machine are checked against the virtual networks available on the host. The host must contain all of the virtual networks required for the virtual machine.

How to Interpret Host Ratings

Host ratings are recommendations based on criteria that you can customize. Remember that host ratings do not include every relevant piece of information. For example, the host rating calculations do not include the speed of network connections.

You can place a virtual machine on a host regardless of its rating if the host has enough physical disk space and memory available. However, placing a virtual machine on a host with a higher rating provides better performance for the virtual machine.

Guidelines for Attaining Accurate Host Ratings

Use the following best practices to help ensure accurate host ratings during placement.

Use Consistent Virtual Network Names

Name virtual networks consistently across hosts. When you create a new virtual machine, Virtual Server automatically creates a virtual network for each network adapter on the host and names the network after the hardware. However, when you migrate virtual machines from one host to another, it is possible to create inconsistent virtual network names. For more information, see “Moving from Virtual Server to Virtual Machine Manager” in the Virtual Machine Manager Operations Guide (

Monitor Trends in Resource Usage

There are three reports that provide useful information about trends in resource usage:

  • Virtual Machine Utilization
  • Host Utilization
  • Host Utilization Growth

You can monitor trends in host and virtual machine usage over time by scheduling periodic reports in Operations Manager 2007. For more information, see the "Report Descriptions" Help topic (

Configure Default Virtual Machine Paths

Configure default virtual machine paths for each volume that you want to make available to virtual machines. Map default virtual machine paths for each volume on which you want to store virtual machines. You must do this mapping if you plan to support virtual machine self-service.


Use the Troubleshooting section to help resolve issues with host placement in Virtual Machine Manager.

When a Host Gets a Zero Rating

You will see a host with a rating of zero if the host does not have the required resources available. Placing a virtual machine on this host exceeds the limits set by the host reserves. Also, you will get a zero rating if, in the placement criteria, the relative importance of all resources is set to zero. On the Host Selection page, click Customize Ratings, and ensure that at least one resource has an importance higher than zero.

You can override the recommendations and place a virtual machine on a host with a zero rating if the host meets physical disk space, memory, and network connectivity requirements. You can find the specific reason for a zero rating by selecting the host in the list and viewing the details beneath the list on the Rating Explanation tab.

When You Cannot Find a Host

If you cannot find a host, consider customizing ratings for the current virtual machine based on the requirements of the virtual machine workload. For some workloads, available CPU might be critical. For others, available disk space might be more important.