Installation guide

Chapter 4.

Virtualization limitations

This chapter covers additional support and product limitations of the virtualization packages in Red Hat

Enterprise Linux 6.

4.1. General limitations for virtualization

Other limitations

For the list of all other limitations and issues affecting virtualization read the Red Hat Enterprise Linux

6 Release Notes. The Red Hat Enterprise Linux 6 Release Notes cover the present new features,

known issues and limitations as they are updated or discovered.

Test before deployment

You should test for the maximum anticipated load and virtualized network stress before deploying

heavy I/O applications. Stress testing is important as there are performance drops caused by

virtualization with increased I/O usage.

4.2. KVM limitations

The following limitations apply to the KVM hypervisor:

Maximum VCPUs per guest

Virtualized guests support up to a maximum of 64 virtualized CPUs in Red Hat Enterprise Linux

6.0.

Constant TSC bit

Systems without a Constant Time Stamp Counter require additional configuration. Refer to

Chapter 14, KVM guest timing management for details on determining whether you have a

Constant Time Stamp Counter and configuration steps for fixing any related issues.

Memory overcommit

KVM supports memory overcommit and can store the memory of guests in swap. A guest will run

slower if it is swapped frequently. When KSM is used, make sure that the swap size is the size of

the overcommit ratio.

CPU overcommit

It is not recommended to have more than 10 virtual CPUs per physical processor core. Any

number of overcommitted virtual CPUs above the number of physical processor cores may cause

problems with certain virtualized guests.

Overcommitting CPUs has some risk and can lead to instability. Refer to Chapter 20,

Overcommitting with KVM for tips and recommendations on overcommitting CPUs.

Virtualized SCSI devices

SCSI emulation is limited to 16 virtualized (emulated) SCSI devices..

Virtualized IDE devices

KVM is limited to a maximum of four virtualized (emulated) IDE devices per guest.