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Planning for Hosts

A virtual machine host is a physical computer that hosts one or more virtual machines.

Host System Configuration

This section provides information that is helpful when planning the system configuration for hosts in your VMM deployment.

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 workloads for the virtual machines running on the host.
  • 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.

Because hosts operate by using Virtual Server 2005, you can refer to the sizing and system configuration information provided in the Virtual Server documentation to help you to size your hosts. Some useful Virtual Server resources are provided below:


The most important resource consideration for a host is the amount of available RAM. After you use VMM to allocate a portion of a host's RAM to a virtual machine, that memory cannot be allocated to other resources in VMM, even if the virtual machine is turned off. In addition, you need to reserve an adequate portion of RAM to run the host operating system and any other applications that are running on the host operating system.

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.


A host can have multiple processors; however, each virtual machine 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

If the virtual machines on a host are running applications that require high availability, you may want to consider using multiple network adaptors. A host that has redundant network adaptors can maintain network connectivity if one network adaptor fails.

You may also want to consider dedicating one network adaptor for exclusive use by the host operating system, with all other network adaptors used by the virtual machine. 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 the "Configuring a SAN Environment for VMM" topic in Deploying VMM (

Maintenance Hosts

You might want to designate one or more hosts as maintenance hosts. A maintenance host is a host that you dedicate for creating and staging virtual machines before you move them into your production environment or for updating virtual machines.

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 the "About Virtual Machine Placement" topic in VMM Help (

You can designate a host as a maintenance host in the placement options of Host Properties. This designation is for reference only. You should also exclude maintenance hosts from virtual machine placement. For more information about setting placement options, see the "How to Set Placement Options for a Host" topic in VMM Help (

Number of Virtual Machines per Host

The number of virtual machines that can be run on a host is primarily limited by the system configuration of the host and of the virtual machines, as is discussed earlier in this topic.

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