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Applies to SUSE Linux Enterprise Server 11 SP4

2 KVM 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.

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 not supported on IBM System z.

User Permissions

The management tools (Virtual Machine Manager, virsh, vm-install) need to authenticate with libvirt—see Chapter 8, 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. We ensure host and VMs install and work successfully, even when reaching the limits and there are no major performance regressions (CPU, memory, disk, network) since the last release.

Max. Guest RAM Size

4 TB

Max. Virtual CPUs per Guest

256

Max. Virtual Network Devices per Guest

8

Max. Virtual Block Devices per Guest

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

Max. Number of VM Guests per VM Host Server

Limit is defined as the total number of virtual CPUs in all guests being no greater than 8 times the number of CPU cores in the host.

The following hardware limits for the host server have been tested.

Max. Physical CPUs

4096

Max. Physical Memory

16 TB

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