Installation guide

Chapter 5. Virtualization limitations
This chapter covers additional limitations of the virtualization packages in Red Hat Enterprise Linux.
5.1. General limit at ions for virt ualizat ion
Co n vert in g b et ween h ypervisors
There is no support for converting Xen-based guests to KVM or KVM-based guests to Xen.
O t h er limit at io n s
See the Red Hat Enterprise Linux Release Notes at https://access.redhat.com/site/documentation/ for
your version. The Release Notes cover the present new features, known issues and limitations as
they are updated or discovered.
T est b ef o re d ep lo ymen t
You should test for the maximum anticipated system and network load before deploying heavy I/O
applications such as database servers. Load testing and planning are important as virtualization
performance can suffer under high I/O.
5.2. KVM limit at ions
The following limitations apply to the KVM hypervisor:
Co n st an t T SC b it
Systems without a Constant Time Stamp Counter require additional configuration. See
Chapter 17, KVM guest timing management to determine whether you have a Constant Time
Stamp Counter and what additional configuration may be required.
Memo ry o verco mmit
KVM supports memory overcommit and can store the memory of guests in swap space. A
guest will run slower if it is swapped frequently. When Kernel SamePage Merging (KSM) is
used, make sure that the swap size is equivalent to the size of the overcommit ratio.
CPU o verco mmit
No support exists for having more than 10 virtual CPUs per physical processor core. A CPU
overcommit configuration exceeding this limitation is unsupported and can cause problems
with some guests.
Overcommitting CPUs has some risk and can lead to instability. See Section 33.4,
“ Overcommitting Resources” for tips and recommendations on overcommitting CPUs.
Virt u aliz ed SCSI d evices
SCSI emulation is presently not supported. Virtualized SCSI devices are disabled in KVM.
Virt u aliz ed IDE d evices
KVM is limited to a maximum of four virtualized (emulated) IDE devices per guest.
Para- virt u aliz ed d evices
⁠Chapt er 5. Virt ualizat ion limit at ions
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