Choosing an optimal server for VM initial placement, migrate, and resume

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

The right-click menu for a VM. The Start on Server submenu is selected. In this submenu, hosts are listed with star ratings out of 5 next to the host name.

More stars appear beside host17 since this server is the optimal host on which to start the VM. host16 does not have any stars beside it, which indicates that the host is not recommended. However, since this host is enabled the user can select that host. host18 is grayed 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 VM. These ratings also apply when you want to start a VM that is powered off or suspended and when you want to migrate the VM to another server.

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 indicate the lowest-rated (least optimal) server. When it is not possible to start or move a VM to a host, the host name is grayed out. The reason that the host cannot accept the VM appears beside it.

In general, Workload Balancing functions more effectively and makes better, less frequent optimization recommendations if you start VMs 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 runs in Maximum Performance mode, Workload Balancing tries to balance the VMs across the hosts in the pool so that all VMs have good performance. When a pool is running in Maximum Density mode, Workload Balancing tries to place VMs onto hosts as densely as possible while ensuring the VMs 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 VMs on hosts with the most resources available of the type the VM requires. In Maximum Density mode, Workload Balancing tries to place VMs on hosts that already have VMs running so that VMs are running on as few hosts as possible.
  • The amount and type of resources the VM requires. After WLB monitors a VM for a while, it uses the VM metrics it gathered to make placement recommendations according to the type of resources the VM requires. For example, WLB might select a host with less available CPU but more available memory if it is what the VM requires (based on its past performance history). However, 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 indicate 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 indicate the most-recommended (optimal) server and five empty stars indicates the least-recommended server.