How Virtual Machine Manager Rates Hosts

Article
04/29/2011

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

When you deploy or migrate a virtual machine to a host, the evaluation and selection of the most suitable host for the virtual machine is known as virtual machine placement, or placement. During placement, Virtual Machine Manager (VMM) evaluates the suitability of available hosts and assigns each host a rating of 0 stars (not suitable) through 5 stars (very suitable), in half-star increments. Each host’s rating is based on several factors, including the virtualization software on the host, the hardware and networking configuration of the host and the virtual machine, the virtual machine's resource requirements, and whether the virtual machine is highly available.

A new feature in VMM 2008 R2 is the host compatibility check, which VMM performs before migrating a virtual machine that is deployed on a Hyper-V host cluster created in Windows Server 2008 R2. To prevent a migration from failing, the host compatibility check ensures that the destination host’s hardware, such as CPU model, is compatible with the source host. VMM uses the Hyper-V compatibility check API in Windows Server 2008 R2 and the VMware compatibility check APIs to ensure that hardware requirements are met for each type of virtualization software. This feature is not available for failover clusters created in Windows Server 2008.

Important

In VMM 2008 R2, for hosts that are running Windows Server 2008 R2 or for ESX Server hosts, the ratings that first appear on the Select Host wizard page are based on a preliminary evaluation by VMM. To see a host rating based on a more thorough evaluation, you must select the host by clicking it.

Using Host Ratings to Select Hosts

You can establish customized default criteria for VMM to use when rating hosts during placement. This helps you to place virtual machines on the most suitable host. You can specify the placement goal that VMM should use to calculate hosts ratings. The placement goals are described in the following table:

Goal	Description	Strategy
Resource maximization	Consolidates multiple low-utilization workloads onto a single host. Virtual machines 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.

You can also specify the relative importance of each of the following resources that VMM should use when rating hosts:

CPU utilization
Memory utilization
Disk I/O
Network utilization

For example, if you create a Windows Server 2008-based virtual machine for developers and you know that the virtual machine requires significant CPU, however the host has relatively little hard disk or network resources, you might customize the default ratings to increase the priority of free CPU while lowering the priority of the hard disk and network.

For more information about setting placement defaults, see How to Set Placement Defaults for Virtual Machines (https://go.microsoft.com/fwlink/?LinkID=162975).

Automatic Placement

VMM automatically places a virtual machine on the most suitable host in a host group and on the most suitable volume on the host based on the volume's available space. This process is known as automatic placement, and it occurs in the following situations:

When you drag a virtual machine onto a host or a host group in Virtual Machines view.
When self-service users deploy virtual machines that they create by using the VMM Self-Service Portal. For more information about self-service, see About Virtual Machine Self-Service (https://go.microsoft.com/fwlink/?LinkID=121834).

Important

A default virtual machine path must be configured on the volume that VMM selects during automatic placement or the virtual machine placement will fail. For more information about how to add default virtual machine paths on hosts, see How to Set Placement Options for a Host (https://go.microsoft.com/fwlink/?LinkID=162976).

During automatic placement, the files for the virtual machine are moved to the most suitable host for a host group, and to the most suitable volume on the selected host based on the volume's available space. For automatic placement to succeed, you must configure a default virtual machine path on the recommended volume. For more information, see About Default Virtual Machine Paths (https://go.microsoft.com/fwlink/?LinkID=162784).

Note

If you configured a virtual machine with the hardware configuration: Make this VM highly available, during placement, VMM will provide host ratings only for clustered Hyper-V hosts and ESX Server hosts that are in a host cluster. For more information, see How to Configure Virtual Machine Priority and Availability.

How VMM Rates Hosts

VMM evaluates all hosts within the selected host group and also any hosts contained in child host groups. The ratings are based on the default placement settings 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

VMM measures CPU usage, memory 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 measurements taken since the last action that changed the host’s resource usage. Actions that reset data on a host include the following:

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, or Saved State

Calculating Host Ratings

VMM calculates host ratings in half-star increments on a scale from 0 stars through 5 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
CPU	[ 1 – ( CPU Utilization / (100 – CPU Reserve)) ] x CPU Weight
Memory	[ 1 – ( Memory Used / (Total Memory – Memory Reserve)) ] x Memory Weight
Disk	[ 1 – ( Disk IOPS / Maximum Disk IOPS ] x Disk Weight
Network	[ 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 hard disk space	The host must have at least one hard disk with enough space to hold the total hard disk space required by the virtual machine. With dynamic hard disks, the current hard disk size is used, not the maximum hard disk size.
Memory	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. Note The host requires slightly more memory to start a virtual machine than it requires to place a virtual machine. If you enable Dynamic Memory on a virtual machine, consider the following placement implications: If the virtual machine (including any one of its checkpoints) is configured to use Dynamic Memory, placement is blocked during creation or migration if the destination Hyper-V host does not support Dynamic Memory. For successful placement of a new or stopped virtual machine, the destination Hyper-V host must meet at least the startup memory requirement of the virtual machine. For successful placement of a running virtual machine, the destination Hyper-V host must meet at least the current memory requirement of the virtual machine. During placement of a virtual machine that is in a saved state, the following values are compared: the last known memory usage value of the virtual machine the startup memory of the virtual machine For successful placement, the destination Hyper-V host must have available whichever value is highest. For more information about Dynamic Memory, see What's New in Virtual Machine Manager 2008 R2 SP1.
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. If you use network tags, the network location and the network tag for the virtual and the host machines must be identical, or the host rating will be 0 stars. For more information, see Configuring Virtual Networks in VMM (https://go.microsoft.com/fwlink/?LinkId=197222).
Video	If you enable the Microsoft RemoteFX 3D video adapter on a virtual machine, consider the following placement implications: If the virtual machine (including any of its checkpoints) is configured to use the RemoteFX 3D video adapter, placement is blocked during creation or migration if the Hyper-V host does not support RemoteFX. For information about the requirements to support RemoteFX, see What's New in Virtual Machine Manager 2008 R2 SP1. During the creation or migration of a running virtual machine, placement is blocked if the destination RemoteFX-capable Hyper-V host does not have one or more RemoteFX-capable graphics processing units (GPUs) with sufficient available video memory. If the virtual machine is stopped or in a saved state, you receive a zero-star rating with a warning. However, placement is not blocked. During the migration of a running virtual machine, placement is blocked if the destination Hyper-V host does not have the same GPU with sufficient available video memory. If the virtual machine is in a saved state, you receive a zero-star rating with a warning. However, placement is not blocked.
Maintenance mode	In VMM 2008 R2, while a host is in maintenance mode, the host receives a zero-star rating and cannot be selected for placement. For more information, see About Maintenance Mode (https://go.microsoft.com/fwlink/?LinkId=178844).
Clustering	Highly available virtual machines must be placed on clustered hosts. When migrating the highly available virtual machine, VMM assigns zero stars to nonclustered hosts and will not block those hosts from placement. If you confirm that you want to migrate the highly available virtual machine to a nonclustered host, the virtual machine is configured as non-highly available. Clustered hosts that are in an Over-committed state receive a zero-star rating but are not blocked from placement. In VMM, the cluster reserve setting specifies the number of node failures a host cluster must be able to sustain while still supporting all virtual machines that are deployed on the hosts. If a host cluster cannot withstand the specified number of node failures and still keep all virtual machines running, the cluster is placed in an Over-committed state. For more information about Over-committed status, see Creating and Managing Highly Available Virtual Machines in VMM (https://go.microsoft.com/fwlink/?LinkID=145066).
Moving from Hyper-V	VMM will block migration of virtual machines from a Hyper-V host to a host that is running different virtualization software.
Virtualization software compatibility	VMM will block migration of a virtual machine with specific features that are not allowed by the virtualization software that is running on a host. For example, VMware hosts do not allow IDE hard disks, and Hyper-V hosts do not allow booting up from a SCSI hard disk.

How to Interpret Host Ratings

Host ratings are recommendations based on default criteria that you can customize or override during placement. 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 hard 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.

Monitor Trends in Resource Usage

You can monitor trends in host and virtual machine usage over time by scheduling periodic reports in Operations Manager. For more information, see Configuring Operations Manager Integration with VMM (https://go.microsoft.com/fwlink/?LinkID=159865).

Configure Default Virtual Machine Paths

Add default virtual machine paths for each volume of a host on which you want to store virtual machines. You must do this if you plan to support virtual machine self-service. For more information about how to add a virtual machine path to the host, see How to Set Placement Options for a Host (https://go.microsoft.com/fwlink/?LinkID=162976). For more information about virtual machine self-service, see About Virtual Machine Self-Service (https://go.microsoft.com/fwlink/?LinkID=121834).

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