Product Documentation

Choosing an Optimal Server for VM Initial Placement, Migrate, and Resume

Oct 09, 2015

When Workload Balancing is enabled and you start a virtual machine that is offline, XenCenter provides recommendations to help you determine the optimal physical host in the resource pool on which to start a virtual machine. The recommendations are also known as star ratings since stars are used to indicate the best host.



More stars appear beside host17 since this is the optimal host on which to start the VM. host16 does not have any stars beside it, which indicates that host is not recommended, but since it is enabled the user can select that host. host18 is greyed out due to insufficient memory, so the user cannot select it.

How do placements work?

When Workload Balancing is enabled, XenCenter provides star ratings to indicate the optimal hosts for starting a virtual machine, including when you want to start the virtual machine when it is powered off or suspended and when you want to migrate the virtual machine to a different server (Migrate and Maintenance Mode).

When you use these features with Workload Balancing enabled, host recommendations appear as star ratings beside the name of the physical host. Five empty stars indicates the lowest-rated (least optimal) server. When it is not possible to start or move a virtual machine to a host, when you use the menu command for one of the placement features, the host name is greyed out and the reason it cannot accept the VM appears beside it.

In general, Workload Balancing functions more effectively and makes better, less frequent optimization recommendations if you start virtual machines on the hosts it recommends (that is, by using one of the placement features to select the host with the most stars beside it).

What does optimal mean?

The term optimal refers to the physical server best suited to hosting your workload. There are several factors Workload Balancing uses when determining which host is optimal for a workload:

  • The amount of resources available on each host in the pool. When a pool is running in Maximum Performance mode, Workload Balancing tries to balance the virtual machines across the hosts in the pool so that all virtual machines have good performance. When a pool is running in Maximum Density mode, Workload Balancing tries to place virtual machines onto hosts as densely as possible while ensuring the virtual machines have sufficient resources.
  • The optimization mode in which the pool is running (Maximum Performance or Maximum Density). When a pool is running in Maximum Performance mode, Workload Balancing tries to place virtual machines on hosts with the most resources available of the type the VM requires. In Maximum Density mode, Workload Balancing tries to place virtual machines on hosts that already have virtual machines running so that virtual machines are running on as few hosts as possible.
  • The amount and type of resources the VM requires. After Workload Balancing has monitored a virtual machine for a while, it uses the VM metrics it gathered to make placement recommendations according to the type of resources the virtual machine requires. For example, Workload Balancing may select a host with less available CPU but more available memory if it is what the VM requires (based on its past performance history). It should, however, be noted that Workload Balancing only makes a recommendation if it determines the current host is under resource pressure.

To start a virtual machine on the optimal server

  1. In the Resources pane of XenCenter, select the virtual machine you want to start.
  2. From the VM menu, select Start on Server and then select one of the following:
    • Optimal Server. The optimal server is the physical host that is best suited to the resource demands of the virtual machine you are starting. Workload Balancing determines the optimal server based on its historical records of performance metrics and your placement strategy. The optimal server is the server with the most stars.
    • One of the servers with star ratings listed under the Optimal Server command. Five stars indicates the most-recommended (optimal) server and five empty stars indicates the least-recommended server.

To resume a virtual machine on the optimal server

  1. In the Resources pane of XenCenter, select the suspended virtual machine you want to resume.
  2. From the VM menu, select Resume on Server and then select one of the following:
    • Optimal Server. The optimal server is the physical host that is best suited to the resource demands of the virtual machine you are starting. Workload Balancing determines the optimal server based on its historical records of performance metrics and your placement strategy. The optimal server is the server with the most stars.
    • One of the servers with star ratings listed under the Optimal Server command. Five stars indicates the most-recommended (optimal) server and five empty stars indicates the least-recommended server.