Chapter 20. Feature support and limitations in RHEL 8 virtualization

This document provides information on feature support and restrictions in Red Hat Enterprise Linux 8 (RHEL 8) virtualization.

20.1. How RHEL 8 virtualization support works

A set of support limitations applies to virtualization in Red Hat Enterprise Linux 8 (RHEL 8). This means that when you use certain features or exceed a certain amount of allocated resources when using virtual machines in RHEL 8, Red Hat will not support these guests unless you have a specific subscription plan.

Features listed in Section 20.2, “Recommended features in RHEL 8 virtualization” have been tested and certified by Red Hat to work with the KVM hypervisor on a RHEL 8 system. Therefore, they are fully supported and recommended for use in virtualization in RHEL 8.

Features listed in Section 20.3, “Unsupported features in RHEL 8 virtualization” may work, but are not supported and not intended for use in RHEL 8. Therefore, Red Hat strongly recommends not using these features in RHEL 8 with KVM.

Section 20.4, “Resource allocation limits in RHEL 8 virtualization” lists the maximum amount of specific resources supported on a KVM guest in RHEL 8. Guests that exceed these limits are not supported by Red Hat.

In addition, unless stated otherwise, all features and solutions used by the documentation for RHEL 8 virtualization are supported. However, some of these have not been completely tested and therefore may not be fully optimized.

Important

Many of these limitations do not apply to other virtualization solutions provided by Red Hat, such as Red Hat Virtualization (RHV) or Red Hat OpenStack Platform (RHOSP).

20.3. Unsupported features in RHEL 8 virtualization

The following features are not supported by the KVM hypervisor included with Red Hat Enterprise Linux 8 (RHEL 8):

Important

Many of these limitations may not apply to other virtualization solutions provided by Red Hat, such as Red Hat Virtualization (RHV) or Red Hat OpenStack Platform (RHOSP).

Features supported by RHV 4.2 and onwards, or RHOSP 13 and onwards, are highlighted as notes in the following document.

  • Guest operating systems

    KVM virtual machines (VMs) using the following guest operating systems on a RHEL 8 host are not supported:

    • Microsoft Windows 8.1 and earlier
    • Microsoft Windows Server 2012 and earlier
    • macOS
    • Solaris for x86 systems
    • Any OS released prior to 2009
    Note

    For a list of guest operating systems supported on RHEL hosts, see Certified guest operating systems for Red Hat Enterprise Linux with KVM.

    For a list of guest operating systems supported by other virtualization solutions provided by Red Hat, see Certified Guest Operating Systems in Red Hat OpenStack Platform and Red Hat Virtualization.

    For a list of guest operating systems supported specifically by RHV, see Supported guest operating systems in RHV .

  • vCPU hot unplug

    Removing a virtual CPU (vCPU) from a running VM, also referred to as a vCPU hot unplug, is not supported in RHEL 8.

    Note

    This feature is supported in RHV. For details, see Hot plugging VCPUs.

  • Memory hot unplug

    Removing a memory device attached to a running VM, also referred to as a memory hot unplug, is unsupported in RHEL 8.

    Note

    This feature is supported in RHV but only on guest VMs running RHEL with specific guest configurations. For details, see Hot Unplugging Virtual Memory.

  • I/O throttling

    Configuring maximum input and output levels for operations on virtual disk, also known as I/O throttling, is not supported in RHEL 8.

    Note

    This feature is supported in RHV. For details, see Storage quality of service.

    This feature is supported in RHOSP. For details, see Setting Resource Limitation on Disk and the Use Quality-of-Service Specifications section in the RHOSP Storage Guide.

  • Storage live migration

    Migrating a disk image of a running VM between hosts is not supported in RHEL 8.

    Note

    This feature is supported in RHV. For details, see Overview of Live Storage Migration.

    This feature is also supported in RHOSP but with some limitations. For details, see Migrate a Volume.

  • Live snapshots

    Creating or loading a snapshot of a running VM, also referred to as a live snapshot, is not supported in RHEL 8.

    In addition, note that non-live VM snapshots are deprecated in RHEL 8. Therefore, creating or loading a snapshot of a shut-down VM is supported, but Red Hat recommends not using it.

    Note

    This feature is supported in RHV with some limitations. For details, see Live snapshots in Red Hat Virtualization.

    This feature is supported in RHOSP. For details, see Importing virtual machines into the overcloud.

  • Vhost-user

    RHEL 8 does not support the implementation of a user-space vHost interface.

    Note

    This feature is supported in RHOSP, but only for virtio-net interfaces. For details, see virtio-net implementation and vhost user ports.

  • S3 and S4 system power states

    Suspending a VM to the Suspend to RAM (S3) or Suspend to disk (S4) system power states is not supported. Note that these features are disabled by default, and enabling them will make your VM not supportable by Red Hat.

    Note

    This feature is supported in RHOSP. For details, see Suspend/Resume Instance in Update a VM instance.

  • S3-PR on a multipathed vDisk

    SCSI3 persistent reservation (S3-PR) on a multipathed vDisk is not supported in RHEL 8. As a consequence, Windows Cluster is not supported in RHEL 8. In case you need Windows Cluster support, use Red Hat Virtualization (RHV) instead.

    Note

    This feature is supported in RHV. For details, see Cluster support on RHV guests.

  • virtio-crypto

    The drivers for the virtio-crypto device are available in the RHEL 8.0 kernel, and the device can thus be enabled on a KVM hypervisor under certain circumstances. However, using the virtio-crypto device in RHEL 8 is not supported and its use is therefore highly discouraged.

    Note

    This feature is not supported in RHV or RHOSP.

Additional resources

20.4. Resource allocation limits in RHEL 8 virtualization

The following limits apply to virtualized resources that can be allocated to a single KVM virtual machine (VM) on a Red Hat Enterprise Linux 8 (RHEL 8) host.

Important

Many of these limitations do not apply to other virtualization solutions provided by Red Hat, such as Red Hat Virtualization (RHV) or Red Hat OpenStack Platform (RHOSP).

  • Maximum VMs per host

    A single RHEL 8 host supports up to 4 VMs running at the same time.

  • Maximum vCPUs per VM

    RHEL 8 supports up to 384 vCPUs allocated to a single VM.

  • PCI devices per VM

    RHEL 8 supports 32 PCI device slots per VM bus, and 8 PCI functions per device slot. This gives a theoretical maximum of 256 PCI functions per bus when multi-function capabilities are enabled in the VM, and no PCI bridges are used.

    Each PCI bridge adds a new bus, potentially enabling another 256 device addresses. However, some buses do not make all 256 device addresses available for the user; for example, the root bus has several built-in devices occupying slots.

  • Virtualized IDE devices

    KVM is limited to a maximum of 4 virtualized IDE devices per VM.

20.5. An overview of virtualization features support

The following tables provide comparative information about the support state of selected virtualization features in RHEL 8 across the supported system architectures.

Table 20.1. Device hot plug and hot unplug

 Intel 64 and AMD64IBM ZIBM POWER

CPU hot plug

Supported

Supported

Supported

CPU hot unplug

UNSUPPORTED

UNSUPPORTED

UNSUPPORTED

Memory hot plug

Supported

UNSUPPORTED

Supported

Memory hot unplug

UNSUPPORTED

UNSUPPORTED

UNSUPPORTED

PCI hot plug

Supported

Supported

Supported

PCI hot unplug

Supported

Supported

Supported

Table 20.2. Other selected features

 Intel 64 and AMD64IBM ZIBM POWER

NUMA tuning

Supported

UNSUPPORTED

Supported

SR-IOV devices

Supported

UNSUPPORTED

Supported

virt-v2v and p2v

Supported

UNSUPPORTED

UNSUPPORTED

Additional sources