Understanding the different virtual machine CPU maximum limits between standalone hosts and host in DRS clusters

Products

• VMware vCenter Server

Issue/Introduction

The maximum CPU limit is different for a standalone host or DRS cluster, depending on where a virtual machine is hosted. This article explains why this CPU limit is different and why it may be overlooked.

Resolution

Standalone

The maximum CPU limit for a standalone host is numVMCPUs * GHzPerHostCore.

Examples: On a physical machine with 8 cores at 2 GHz each :

• A virtual machine with 2 vCPUs has a maximum limit of 4GHz: 2 vCPUs * 2GHz per core
• The virtual machine could have up to 8 vCPUs, when it would have a maximum CPU limit of 16GHZ: 8 vCPUs * 2GHz per core

DRS Cluster

A DRS cluster can be formed with heterogeneous hosts. Virtual machines can be migrated to any host with different physical core speeds, because the resources in this cluster can vary with the addition or subtraction of hosts. The maximum value of the CPU slider is the cluster's total CPU capacity. This value indicates that virtual machines may run on any host in the cluster and that you are abstracting the resources at a cluster level rather than a single physical host level.

The maximum CPU limit for a DRS cluster is the sum of numVMCPUs * GHzPerHostCore for each host.

Example: In a DRS cluster formed by an 8-core host, each at 2GHz, and a 4-core host, each at 2.5 GHz, a virtual machine could potentially have 12 vCPUs with a a maximum CPU limit of 26GHz: (8 vCPUs * 2GHz) + (4 vCPUs * 2.5 GHz) = 16 GHz + 10 GHz = 26 GHz.

Note: The actual value you see may be a little less than 26GHz due to virtualization overhead.