KVM Limitations

Contents

2.1. General Limitations
2.2. Hardware Limitations
2.3. Performance Limitations

Although virtulized machines behave almost like physical machines some limitations apply. These affect both, the guest as well as the host system.

General Limitations

The following general restrictions apply when using KVM:

Overcommits

KVM allows for both memory and disk space overcommit. It is up to the user to understand the implications of doing so however, as hard errors resulting from actually exceeding available resources will result in guest failures. CPU overcommit is also supported but carries performance implications.

Time Synchronization

Most guests require some additional support for accurate time keeping. Where available, kvm-clock is to be used. NTP or similar network based time keeping protocols are also highly recommended (for host and guest) to help maintain a stable time. When using the kvm-clock running NTP inside the guest is not recommended.

MAC addresses

If no MAC address is specified for a NIC, a default MAC address will be assigned. This can result in network problems when more than one nick receives the same MAC address. It is recommended to always specify a unique MAC address for each NIC.

Live Migration

Live Migration is only possible between hosts with a processor from the same vendor. Guest storage has to be accessible from both hosts.

User permissions

The management tools (virt-manager, vm-install) can only be used by the user root. In order to invoke qemu-kvm from the command line, a user has to be a member of the group kvm.

Suspending/Hibernating the Host System

Suspending or hibernating the host system while guests are running is not supported.

Hardware Limitations

The following limits hardware limits for guests have been tested, and are supported:

max. Guest RAM size

512 GB

max. Virtual CPUs per guest

16

max. NICs per guest

8

max. Block devices per guest

4 emulated, 20 para-virtual (using virtio-blk)

max, number of guests

no more than 8 times the number of cpu cores in the host

Performance Limitations

Basically workloads designed for physical installations can be virtualized and therefore inherit the benefits of modern virtualization techniques. However, virtualization comes at the cost of a slight to moderate performance impact. You should also always test your workload with the maximum anticipated CPU and I/O load to verify if it is suited for being virtualized. Although every reasonable effort is made to provide a broad virtualization solution to meet disparate needs, there will be cases where the workload itself is unsuited for kvm virtualization.

We therefore propose the following performance expectations for guests performance to be used as a guideline. The given percentage values are a comparison of performance achieved with the same workload under non-virtualized conditions. The values are rough approximations and cannot be guaranteed.

Category

Fully Virtualized

Paravirtualized

Host-Passthrough

CPU, MMU

7%

not applicable

97% (Hardware Virtualization with Extended Page Tables(Intel) or Nested Page Tables (AMD)
85% (Hardware Virtualization with shadow page tables)

Network I/O (1GB LAN)

20% (Realtek emulated NIC)

75% (virtio-net)

95%

Disk I/O

40% (IDE emulation)

85% (virtio-blk)

95%

Graphics (non-accelerated)

50% (VGA or Cirrus)

not applicable

not applicable

Time accuracy (worst case, using recommended settings without NTP)

95% - 105% (where 100% = accurate)

100% (kvm-clock)

not applicable