Chapter 2. KVM Limitations

Chapter 2. KVM Limitations¶

Contents

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

Although virtualized machines behave almost like physical machines, some limitations apply. These affect both, the VM Guest as well as the VM Host Server system.

2.1. 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, hard errors resulting from 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 VM Host Server and VM Guest) to help maintain a stable time. Running NTP inside the guest is not recommended when using the kvm-clock . Refer to Section 9.4, “Clock Settings” for details.
MAC addresses: If no MAC address is specified for a NIC, a default MAC address will be assigned. This may result in network problems when more than one NIC receives the same MAC address. It is recommended to always assure a unique MAC address has been assigned for each NIC.
Live Migration: Live Migration is only possible between VM Host Servers with a processor from the same vendor. Guest storage has to be accessible from both VM Host Servers.
User Permissions: The management tools (Virtual Machine Manager, virsh, vm-install) need to authenticate with libvirt—see Chapter 7, Connecting and Authorizing for details. In order to invoke qemu-kvm from the command line, a user has to be a member of the group kvm.
Suspending/Hibernating the VM Host Server: Suspending or hibernating the VM Host Server system while guests are running is not supported.

2.2. Hardware Limitations¶

The following virtual hardware limits for guests have been tested:

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 VM Host Server

2.3. 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 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.