Host Overcommitment

Before allocating a VM to a Host, the Scheduler checks that the capacity requested by the VM fits in the available capacity of the Host. The overall number of VMs assigned to a Host can be controlled by:

  • Adjusting the total capacity announced by each Host.
  • Adjusting the capacity requested by the VM.

Virtual Machine Capacity

The resource allocation of the VM is expressed with the following attributes:

  • CPU, percentage of CPU divided by 100 required for the VM, e.g. 0.5 will request half of a processor.
  • MEMORY, total memory in MB.

It is important to not confuse with VCPU which is the number of virtual processors the Guest OS will see. Note also that the hypervisor is configured to follow the CPU allocations of the VM. In this way a VM with CPU=0.5 will tend to use half of the CPU cycles of a VM with CPU=1.0.

For example, a Host with 8 processors (TOTAL CPU=800) can host eight VMs with CPU=1 or 16 VMs with CPU=0.5. In this way CPU can be use to pack more VMs in a Host.

Host Capacity

The capacity of a Host is obtained by the monitor probes. You may alter the overall resources available to the VMs by reserving an amount of that capacity:

  • Cluster-wise, by updating the cluster template (e.g. onecluster update). All the host of the cluster will reserve the same amount of capacity.
  • Host-wise, by updating the host template (e.g. onehost update). This value will override those defined at cluster level.

In particular the following capacity attributes can be reserved:

Attribute Description
RESERVED_CPU (CPU percentage) It will be subtracted from the TOTAL CPU. See scheduler.
RESERVED_MEM (KB). It will be subtracted from the TOTAL MEM. See scheduler.

Important

These values can be negative, in that case you’ll be actually increasing the overall capacity so overcommiting host capacity.

The above values can be absolute, for example RESERVED_MEM=-10240 will add 1GB of memory to the Host. Or they can be expressed in percentage, for example RESERVED_MEM=-10% will increase the memory of the host by a 10%.

Note also that the available storage capacity in a System Datastore is also checked before allocating a VM to it. However, you cannot overcommit this capacity.