How Virtual Machine Manager Rates Hosts
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.
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:
|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
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:
|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:
|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.|
|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.|
|Clustering||Highly available virtual machines should be placed on clustered hosts. VMM will assign 0 stars for a non-clustered host but will not block the action. You should also ensure that migrations on the same cluster do not violate the cluster reserve limits. For more information, see How to View and Modify the Properties of a Host Cluster (http://go.microsoft.com/fwlink/?LinkID=162986).|
|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 (http://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. For more information about virtual machine self-service, see About Virtual Machine Self-Service.