Installation guide

Chapter 4. Virtualization restrictions
This chapter covers additional support and product restrictions of the virtualization packages in Red
Hat Enterprise Linux 6.
4.1. KVM rest rict ions
The following restrictions apply to the KVM hypervisor:
Maximu m vCPUs p er g u est
The maximum amount of virtual CPUs that is supported per guest varies depending on
which minor version of Red Hat Enterprise Linux 6 you are using as a host machine. The
release of 6.0 introduced a maximum of 64, while 6.3 introduced a maximum of 160.
Currently with the release of 6.6, a maximum of 160 virtual CPUs per guest is supported.
Co n st an t TSC b it
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.
Memo ry overcommit
KVM supports memory overcommit and can store the memory of guest virtual machines in
swap. A virtual machine will run slower if it is swapped frequently. Red Hat Knowledgebase
has an article on safely and efficiently determining an appropriate size for the swap
partition, available here: https://access.redhat.com/site/solutions/15244. When KSM is used
for memory overcommitting, make sure that the swap size follows the recommendations
described in this article.
Important
When device assignment is in use, all virtual machine memory must be statically pre-
allocated to enable DMA with the assigned device. Memory overcommit is therefore
not supported with device assignment.
CPU o verco mmit
It is not recommended to have more than 10 virtual CPUs per physical processor core.
Customers are encouraged to use a capacity planning tool in order to determine the CPU
overcommit ratio. Estimating an ideal ratio is difficult as it is highly dependent on each
workload. For instance, a guest virtual machine may consume 100% CPU on one use case,
and multiple guests may be completely idle on another.
Red Hat does not support running more vCPUs to a single guest than the amount of overall
physical cores that exist on the system. While Hyperthreads can be considered as cores,
their performance can also vary from one scenario to the next, and they should not be
expected to perform as well as regular cores.
Refer to the Red Hat Enterprise Linux Virtualization Administration Guide for tips and
recommendations on overcommitting CPUs.
Virt u aliz ed SCSI d evices
Red Hat Ent erp rise Linux 6 Virt ualiz at ion Host Configurat ion and G uest Inst allat ion G uide
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