Performance Tuning

Overview

This page explains how to tune VM performance in Private Cloud Director (PCD) by setting image properties. The recommendations listed below should yield positive results for the vast majority of use cases, but the actual outcome will depend on factors like NUMA node topology and the workloads running within the VM.

Note: Most image property changes apply only to new VMs created after making the update. For existing VMs, plan to rebuild the VM to see the effect.

Recommended Image Properties

To update image properties, navigate to Virtual Machines in the left-hand navigation menu > Images & VM Snapshots > Images tab. Select an image and click on Edit Properties. You can enter one or more of the configurations from this section in a key-value format. Click on Update Properties after you complete your updates.

OS Type

The os_type key indicates what OS is contained in the image.

Values

Set the value to one of the following, depending on the image OS.

• linux

• windows

Please note that the January 2026 release of PCD includes a required drop-down for setting this value in the new image upload wizard. You only need to set this value manually for images uploaded prior to the January 2026 release.

Machine Type

The hw_machine_type key indicates the virtualized chipset used by VMs. Set the value to q35 to use the newer Q35 chipset for modern x86 workloads.

Values

PCD supports two main variants of machine type for x86 hosts:

• pc, which corresponds to Intel's I440FX chipset (released in 1996)

• q35, which corresponds to Intel's 82Q35 chipset (released in 2007)

The pc machine type is considered legacy, and does not support many modern features. Some long-term stable Linux distributions (CentOS, RHEL, possibly others) are moving to support q35 only.

Paravirtualized I/O

Disk Controller

The hw_disk_bus key sets the virtual disk controller type that the VM will see when it boots from the image. Setting the value to virtio presents a paravirtualized disk device, which generally yields better performance than fully emulated legacy controllers.

Value

• virtio attaches disks using a paravirtualized controller/device model.

Virtual Network Interface Model

The hw_vif_model key sets the virtual network adapter model exposed to the guest operating system for VMs created from the image. Setting it to virtio presents a paravirtualized network device, which is commonly used for better network performance than fully emulated adapters.

Value

• virtio uses a paravirtualized network adapter model.

QEMU Guest Agent

The hw_qemu_guest_agent controls whether QEMU Guest Agent support is enabled for VMs created from an image. When set to yes, the compute host can communicate with the guest operating system through a QEMU Machine Protocol (QMP) socket (assuming the QEMU guest agent is installed and running inside the VM). Set it to no to explicitly disable guest agent support.

Values

• yes to enable QEMU guest agent communication via QMP socket

• no to disable guest agent support