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 not to confuse this 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 the CPU cycles of a VM with CPU=1.0.

For example, a Host with eight processors (TOTAL CPU=800) can host eight VMs with CPU=1 or 16 VMs with CPU=0.5. In this way, CPU can be used 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 Hosts in 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 the cluster level.

In particular, the following capacity attributes can be reserved:

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

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