第 15 章 Attaching host devices to virtual machines

Just like physical machines, virtual machines (VMs) require specialized devices to provide functions to the system, such as processing power, memory, storage, networking, or graphics. Physical systems usually use hardware devices for these purposes. However, because VMs work as software processes, they need to use software abstractions of such devices instead, referred to as virtual devices.

The basics of virtual devices

Virtual devices attached to a VM can be configured when creating the VM, and can also be managed on an existing VM. Generally, virtual devices can be attached or detached from a VM only when the VM is shut off, but some can be added or removed when the VM is running. This feature is referred to as device hot plug and hot unplug.

When creating a new VM, libvirt automatically creates and configures a default set of essential virtual devices, unless specified otherwise by the user. These are based on the host system architecture and machine type, and usually include:

To manage virtual devices after the VM is created, use the command line (CLI). However, to manage virtual storage devices and network interfaces, you can also use the RHEL 10 web console.

Performance or flexibility

For some types of devices, RHEL 10 supports multiple implementations, often with a trade-off between performance and flexibility.

For example, the physical storage used for virtual disks can be represented by files in various formats, such as qcow2 or raw, and presented to the VM by using a variety of controllers:

An emulated controller is slower than a virtio controller, because virtio devices are designed specifically for virtualization purposes. However, emulated controllers make it possible to run operating systems that have no drivers for virtio devices. Similarly, virtio-scsi offers a more complete support for SCSI commands, and makes it possible to attach a larger number of disks to the VM. Finally, virtio-blk provides better performance than both virtio-scsi and emulated controllers, but a more limited range of use cases. For example, attaching a physical disk as a LUN device to a VM is not possible when using virtio-blk.

For more information about types of virtual devices, see Types of virtual devices.