Live migration¶

If the host capabilities used by the VM on host 1 are also present on host 2, you can perform a live migration from host 1 to host 2.

Choose carefully all XML tags related to the host architecture (<vcpu>, <cputune>, <cpu>, and so on).

For <cpu> XML tags, use the mode="host-model" option only if all hosts are strictly identical.

Note

To get common capabilities between hosts, use the virsh capabilities or cpu-baseline commands.

Example

We will get common capabilities between a Sandybridge host and a Westmere host.

On the first host (Sandybridge), get the CPU capabilities:
```
$ virsh capabilities > host1.xml
```

On the second host (Westmere), get the common capabilities between the two hosts to set in the <cpu> XML tag:

$ virsh cpu-baseline host1.xml
<cpu mode='custom' match='exact'>
  <model fallback='forbid'>Westmere</model>
  <vendor>Intel</vendor>
  <feature policy='require' name='invtsc'/>
  <feature policy='require' name='rdtscp'/>
  <feature policy='require' name='pdpe1gb'/>
  <feature policy='require' name='dca'/>
  <feature policy='require' name='pcid'/>
  <feature policy='require' name='pdcm'/>
  <feature policy='require' name='xtpr'/>
  <feature policy='require' name='tm2'/>
  <feature policy='require' name='est'/>
  <feature policy='require' name='smx'/>
  <feature policy='require' name='vmx'/>
  <feature policy='require' name='ds_cpl'/>
  <feature policy='require' name='monitor'/>
  <feature policy='require' name='dtes64'/>
  <feature policy='require' name='pclmuldq'/>
  <feature policy='require' name='pbe'/>
  <feature policy='require' name='tm'/>
  <feature policy='require' name='ht'/>
  <feature policy='require' name='ss'/>
  <feature policy='require' name='acpi'/>
  <feature policy='require' name='ds'/>
  <feature policy='require' name='vme'/>
</cpu>

Migration may fail due to restrictions on which flags can be migrated. In the example above, we must remove the invtsc feature, since it cannot be migrated.

The following patch solves this issue, but it is not integrated in libvirt yet:

http://www.redhat.com/archives/libvir-list/2015-March/msg01521.html

You can migrate a VM from host 1 to host 2, via virsh migrate:

$ virsh migrate $domain_name --live \
         qemu+ssh://$name_or_address_of_host2/system \
         tcp://$name_or_address_of_host2