IBM Z hardware CPU model
The destination host must offer the same or a later CPU model than the CPU model that is used on the original host.
You can perform virtual server live migrations across IBM Z® hardware of the same model and upgrade level. You can also migrate to a later hardware model, for example from IBM® z15™ to IBM z16 hardware.
Migration to a prior hardware model is possible only if the virtual server on the newer hardware is restricted to CPU features that are also available on the older destination hardware. By default, a virtual server uses the latest CPU model of the hardware. Use the <cpu> element in the domain XML to configure a specific backlevel CPU model.
After a live migration to a later hardware model, the virtual server keeps running with the CPU model of the original hardware. This behavior preserves the option for a live migration back to the original hardware. To use new CPU features on the destination hardware, stop the virtual server, modify the domain configuration-XML, and then restart the virtual server.
Regardless of the destination hardware, live guest migration is possible only to KVM hosts with CPU model support.
If you cannot perform live guest migration, migrate by shutting down the virtual server and then starting it on the destination hardware.
Example
The following figure illustrates the rules for a live migration. A virtual server on z15 hardware runs a guest operating system with the CPU features of z14 with upgrade level 2.
- Guest live migration is possible to IBM z16 hardware.
- Guest live migration is possible to z14 hardware with upgrade level 2.
- Guest live migration is not possible to z14 hardware with less than upgrade level 2.
Confirming the CPU model of a destination host
Use the virsh hypervisor-cpu-compare command to confirm that the CPU model of a particular destination host satisfies the CPU requirements of a virtual server.
As command input, use an XML document that describes the CPU requirements of the virtual server that you want to migrate. For example, use the output of the domcapabilities command.
Run the hypervisor-cpu-compare command on the destination host.
Establishing a baseline CPU model
Use the virsh hypervisor-cpu-baseline command to establish a CPU model that is supported across a set of KVM hosts. The CPU requirements for any virtual server that uses this baseline model are then met by all hosts within the set.
As command input, use an XML document that contains a concatenation of CPU model descriptions, one for each host in the set. Descriptions must adhere to the syntax of the <cpu> element of a domain configuration-XML. To obtain a valid CPU description for a particular KVM hypervisor, issue the domcapabilities command on that hypervisor. Concatenate the output of the domcapabilities command from all KVM hypervisors for which you want to find a baseline CPU model. Optionally, you can reduce the information for each hypervisor to the CPU snippet.