Moving VMs

The prerequisites for this recipe are quite extended.

For the host, the following are the requirements:

The requirement for the guest is:

There are multiple ways to migrate hosts, but we will only highlight the two most common ways.

~]# virsh migrate --domain <guestname> --live –-persistent --undefinesource --verbose --desturl qemu+ssh://<host 2>/system

~]# virsh migrate --domain <guestname> --live –-persistent --undefinesource --verbose --desturl qemu+ssh://<host 2>/system tcp://<local ip address on dedicated network>/

This type of migration is called a "hypervisor native" transport. The biggest advantage of this type of migration is that it incurs the lowest computational cost by minimizing the number of data copies involved.

When we migrate a host, it performs a copy of the memory of the guest to the new host. When the copying is successful, it kills the guest on the source host and starts it on the new host. As the memory is copied, the interruption will be very short-lived.

Communication between the two hosts is over SSH, which is already pretty secure. However, it's also possible to tunnel the data over an even more strongly encrypted channel by specifying the --tunnelled option. This will impose more traffic on your network as there will be extra data communication between the two hosts.

The --compress option can help you out if you wish to reduce the traffic over your network, but this will increase the load on both your hosts as they need to compress/decompress the data, which, in turn, may impact your guests performance. If time is not of the essence but traffic is, this is a good solution.

There's very good and in-depth documentation about this process at https://libvirt.org/migration.html.