Planning for Hosts and Host Groups

Applies To: Virtual Machine Manager 2008, Virtual Machine Manager 2008 R2, Virtual Machine Manager 2008 R2 SP1

A virtual machine host is a physical computer that hosts one or more virtual machines. This topic provides information that can be helpful when planning the sizing, system configuration, number, and locations of hosts in your Virtual Machine Manager (VMM) 2008 or VMM 2008 R2 environment.

Host Types in VMM

VMM supports the following types of hosts:

  • Windows Server–based hosts that are located in an Active Directory Domain Services (AD DS) domain that has two-way trust with the VMM server’s AD DS domain.

  • Windows Server–based hosts that are located in an AD DS domain that does not have a two-way trust with the VMM server’s AD DS domain.

  • Windows Server–based hosts that are located on a perimeter network.

  • Windows Server–based hosts that are in a disjointed namespace, where the host’s fully qualified domain name (FQDN) resolved from the domain name service (DNS) is not the same as name obtained from AD.

  • VMware ESX Server hosts located anywhere in your environment.


    Before you can add ESX Server hosts, you must add a VMware VirtualCenter Server to VMM. When you add a VMware VirtualCenter Server to VMM, all existing ESX Server hosts managed by that VirtualCenter Server are imported to VMM. For more information about adding a VMware VirtualCenter Server, see Configuring VMM to Manage a VMware Infrastructure 3 Environment.

Host Groups

You can create custom groups of virtual machine hosts, known as host groups, for grouping hosts and their virtual machines in meaningful ways. For example, you might create a host group for each branch office in your organization. You can also use host groups to set aside resources on hosts in the host group for the use of the host operating system. Host groups are represented by folders in the navigation pane of the Hosts view and Virtual Machines view. For more information about how you can use host groups, see About Host Groups.

Host Locations

To enhance performance and reduce network traffic during virtual machine creation, it’s important to have your hosts located close to the library servers where you store the files that you use to create virtual machines. For geographically-disperse organizations, you may want to locate hosts and library servers at each branch office or at other remote locations and managed by a centralized VMM server. In this way, users in those locations can build virtual machines by using resources from a local library server instead of copying multi-gigabyte files from a centralized library server over a wide area network (WAN). For more information, see Planning for Branch Offices or Remote Sites.

If you connect to a library server from hosts across a LAN network, your library server should be as close to the hosts as possible on the network. It is a best practice to connect all computers in a VMM configuration with at least a 100-MB Ethernet connection. Using a gigabit Ethernet connection will improve performance especially when combined with a more powerful processor than the recommended processor on the VMM server.

If you use a SAN, it is a best practice to have a library server on the same SAN as the virtual machine hosts that use the library server. By doing so, the library server and the hosts can all access the same logical unit numbers (LUNs) on the SAN, which allows you to make faster file transfers. For more information, see Configuring a SAN Environment for VMM.

Host System Configuration and Resources

This section provides information that can be helpful when planning the sizing and system configuration of the hosts in your VMM environment. You might also want to refer to the sizing and system configuration information provided in the documentation of the virtualization software running on your hosts.

The performance of virtual machines running on a host is greatly impacted by the amount of host resources that are allocated and in use by the virtual machines, and by the host resources that have been set aside for the host operating system, known as host reserves. The host group determines the default host reserves. For more information about default host reserves, see About Host Groups.

Determining the appropriate system configuration for a virtual machine host depends on a number of factors, including but not limited to:

  • The number and type of guest operating systems running on the host.

  • The system configurations of the virtual machines running on the host.

  • The types of applications running on the guest operating systems.

  • The network needs of the host, such as whether it will use a storage area network (SAN) or the local area network (LAN) to make file transfers.


One of the most important resource considerations for a host is the amount of available RAM. When you start a virtual machine, VMM allocates a portion of a host's RAM to that virtual machine based on the virtual machine’s configuration. Once allocated by VMM, that portion of RAM is dedicated to the virtual machine and therefore unavailable for use by other virtual machines on the host or by the host’s operating system. You cannot start a virtual machine if doing so would cause the allocation of RAM to exceed the available RAM on the host.

Storage Space

Whether you use direct-attached storage (DAS) or a storage area network (SAN), it is important that a host have adequate storage space. You need to provide adequate space for each virtual machine running on the host, as well as for the host. You must also take into account the extra space needed for:

  • Each virtual machine's paging file.

  • Dynamically expanding virtual hard disks.

  • Saving the contents of each virtual machine's RAM when putting the virtual machine into a saved state.

  • Virtual machine checkpoints.

If you use DAS, you may want to use multiple hard disks with a limited number of virtual machines running on each hard disk, especially if you are running an application on the virtual machines that has a large number of read and write operations, such as SQL Server. It is a best practice to run virtual machines on a separate hard disk than the host operating system to reduce conflicts of the input/output needs between the host and virtual machines.


For Hyper-V hosts:

Each virtual machine on a host running Hyper-V can have up to 4 processors.

For Virtual Server hosts:

A host running Virtual Server can have multiple processors; however, each virtual machine on a Virtual Server host runs as a single processor computer regardless of how many processers are on the physical computer. You cannot specify which processor a specific virtual machine should use.

Provided there are adequate levels of other resources, having more processors on a host enables you to run more virtual machines on that host. You can also run more virtual machines than there are host processors, but this can slow performance of the virtual machines.

Network Adapters

You should dedicate one network adapter for exclusive use by the host operating system, with all other network adapters used by the virtual machines. Isolating a host's network connection keeps it from being encumbered with high traffic from the virtual machines.

If you use either a Fibre Channel or iSCSI SAN, you can reduce the impacts on your network by doing SAN transfers in place of transfers over the network. When you make a SAN transfer, the logical unit number (LUN) containing the virtual machine is remapped from the source computer to the destination computer instead of transferring the files over the network. Therefore, SAN transfers are much faster than standard network transfers and are independent of the size of the files being transferred. For more information about using VMM in a SAN environment, see Configuring a SAN Environment for VMM.

Supported Number of Hosts and Virtual Machines

You must add at least one host to your VMM environment. You can use a single computer as both the VMM server and as a host; however, this is recommended only if you are managing a small number of hosts. For better performance when you are managing a large number of hosts, it is recommended that you distribute workloads by installing individual VMM components on dedicated computers.

The number of virtual machines that can be run on a host is limited by the configuration of the host and of the virtual machines on the host. For more information about the supported hardware configurations for virtual machines, see Supported Hardware Capabilities.

The maximum number of hosts and virtual machines tested with and supported by VMM on the highest recommended hardware configuration is 400 hosts and 8,000 virtual machines.

If your VMM implementation has over 150 hosts, we strongly recommended that you enable server-optimized garbage collector (GC) on the VMM server instead of the default workstation garbage collector. This can significantly reduce the CPU utilization on the VMM server and improve your performance for parallel VMM operations.

To enable server-optimized garbage collector (GC) on the VMM server, create a file that is named vmmservice.exe.config by using the following code, and then for VMM 2008, place it into the %SYSTEMDRIVE%\Program Files\Microsoft System Center Virtual Machine Manager 2008\Bin directory on the VMM server, or for VMM 2008 R2, place it into the %SYSTEMDRIVE%\Program Files\Microsoft System Center Virtual Machine Manager 2008 R2\Bin directory on the VMM server.

      <gcServer enabled="true"/>

Maintenance Hosts

To optimize the performance of virtual machine hosts, it is recommended that you dedicate one or more hosts to be used as maintenance hosts. A maintenance host is a host that you use only for virtual machine maintenance tasks, such as patching stored virtual machines and templates or staging scripted virtual machine creation before you move the virtual machines into your production environment. By using a dedicated host to perform virtual machine maintenance tasks, you can avoid affecting the performance and host rating of a production host while you perform such tasks.


A maintenance host should not be confused with a host that is in maintenance mode. In VMM 2008 R2, you start maintenance mode temporarily on a host to perform maintenance tasks to the physical host, and you designate a maintenance host to perform ongoing maintenance tasks to virtual machines. For more information about maintenance mode, see About Maintenance Mode.

When you create a new virtual machine on a host, or start an existing virtual machine on a host to apply updates or to perform other maintenance tasks, a portion of the host's physical resources are used by the virtual machine. Depending on the system configuration of the virtual machine and what other virtual machines or applications are running on the host, the host's performance can be degraded. This degradation can also temporarily affect the host's rating for automatic placement of virtual machines. For more information about placement, see About Virtual Machine Placement.

You should remove a designated maintenance host from placement so that it will not be included in the host ratings during the placement of virtual machines. For more information about removing a host from placement, see How to Set Placement Options for a Host.

Custom Host Properties

The host properties provide you with 10 user-configurable custom fields that you can use in any manner you want. For example, you might use custom fields to identify, track, and sort hosts into various categories, such as by department, by geographic area, or by function (for example, development, test, or production). One example of how this feature might be helpful is if you need to track the use of virtual machines for the purposes of allocating the usage costs back to the end-user departments.

See Also


Planning Your VMM Deployment
Planning for the VMM Library
Planning for Branch Offices or Remote Sites