When creating a VM, you can specify the cluster node to host it or choose to assign a node automatically. The node selection algorithm depends on the cluster settings. 

When you create a cluster, you choose the principle of VM distribution on nodes — homogeneous or successive. In successive distribution, VMs are created on the cluster node that is filled up the most, in homogeneous distribution — on the most vacant node.

To select the most vacant or the most filled up node in a cluster, VMmanager calculates the available resources rate for each node.

The rate is calculated as follows: (T−U)÷T. 

T — total mount or resources on the cluster node, taking into account overselling;

U — actual resource usage. 

How the system selects a node:

  1. Calculates the rate for every node: 
    1. the rate by resource types: CPU number, RAM, and disk space.
    2. chooses the smallest result of three
  2. Groups the cluster nodes by the rates. In successive distribution — the lower the rate, the higher the priority of node selection. In homogeneous distribution — the higher the rate, the higher the priority of node selection.
  3. Checks that a VM can be located on the cluster node. If the node has insufficient resources, the system will check the next one by priority. 
  4. The process will stop when the system finds the node with sufficient resources.