13.2. Types of virtual devices
Virtualization in RHEL 9 can present several distinct types of virtual devices that you can attach to virtual machines (VMs):
- Emulated devices
Emulated devices are software implementations of widely used physical devices. Drivers designed for physical devices are also compatible with emulated devices. Therefore, emulated devices can be used very flexibly.
However, since they need to faithfully emulate a particular type of hardware, emulated devices may suffer a significant performance loss compared with the corresponding physical devices or more optimized virtual devices.
The following types of emulated devices are supported:
- Virtual CPUs (vCPUs), with a large choice of CPU models available. The performance impact of emulation depends significantly on the differences between the host CPU and the emulated vCPU.
- Emulated system components, such as PCI bus controllers.
- Emulated storage controllers, such as SATA, SCSI or even IDE.
- Emulated sound devices, such as ICH9, ICH6 or AC97.
- Emulated graphics cards, such as VGA cards.
- Emulated network devices, such as rtl8139.
- Paravirtualized devices
Paravirtualization provides a fast and efficient method for exposing virtual devices to VMs. Paravirtualized devices expose interfaces that are designed specifically for use in VMs, and thus significantly increase device performance. RHEL 9 provides paravirtualized devices to VMs using the virtio API as a layer between the hypervisor and the VM. The drawback of this approach is that it requires a specific device driver in the guest operating system.
It is recommended to use paravirtualized devices instead of emulated devices for VM whenever possible, notably if they are running I/O intensive applications. Paravirtualized devices decrease I/O latency and increase I/O throughput, in some cases bringing them very close to bare-metal performance. Other paravirtualized devices also add functionality to VMs that is not otherwise available.
The following types of paravirtualized devices are supported:
-
The paravirtualized network device (
virtio-net). Paravirtualized storage controllers:
-
virtio-blk- provides block device emulation. -
virtio-scsi- provides more complete SCSI emulation.
-
- The paravirtualized clock.
-
The paravirtualized serial device (
virtio-serial). -
The balloon device (
virtio-balloon), used to dynamically distribute memory between a VM and its host. -
The paravirtualized random number generator (
virtio-rng).
-
The paravirtualized network device (
- Physically shared devices
Certain hardware platforms enable VMs to directly access various hardware devices and components. This process is known as device assignment or passthrough.
When attached in this way, some aspects of the physical device are directly available to the VM as they would be to a physical machine. This provides superior performance for the device when used in the VM. However, devices physically attached to a VM become unavailable to the host, and also cannot be migrated.
Nevertheless, some devices can be shared across multiple VMs. For example, a single physical device can in certain cases provide multiple mediated devices, which can then be assigned to distinct VMs.
The following types of passthrough devices are supported:
- USB, PCI, and SCSI passthrough - expose common industry standard buses directly to VMs in order to make their specific features available to guest software.
- Single-root I/O virtualization (SR-IOV) - a specification that enables hardware-enforced isolation of PCI Express resources. This makes it safe and efficient to partition a single physical PCI resource into virtual PCI functions. It is commonly used for network interface cards (NICs).
- N_Port ID virtualization (NPIV) - a Fibre Channel technology to share a single physical host bus adapter (HBA) with multiple virtual ports.
- GPUs and vGPUs - accelerators for specific kinds of graphic or compute workloads. Some GPUs can be attached directly to a VM, while certain types also offer the ability to create virtual GPUs (vGPUs) that share the underlying physical hardware.
