View source for Migrating Xen domainU Guests Between Host Systems

One of the most compelling features of Xen virtualization is the ability to migrate a domainU guest from one host system to another. of particular significance is Xen's ability to perform live migrations whereby a running guest domainU system is moved from one system to another such that there is only an imperceptible interruption in service.

== Requirements for Xen domainU Migration ==

Before a Xen guest system can be migrated from one host to another a number of requirements must be met:

* Both the source and destination hosts must have access to the root filesystem (and swap is specified) via the same path name. For example if the root filesystem is contained in a disk image with a path of /xen/xenguest.img then that image file must also be accessible at the same location on the target host. This is most commonly achieved by placing the image files on a file server such that it can be mounted via NFS.

* Both systems must be running compatible processors. 

* The target host must have sufficient memory to accommodate the migrated guest domain.

* The source and destination machines must reside on the same subnet.

* The two systems need to be running compatible versions of Xen.

* Both systems must be configured to allow migration of virtual machines (see below).

As long as the above requirements are met then you are ready to perform a Xen guest migration.

== Enabling Xen Guest Migration ==

By default guest domain migration is disabled in the Xen configuration. A number of changes are necessary, therefore, prior to performing migrations. The required settings are located in the ''/etc/xen/xend-config.sxp'' configuration file and need to be implemented on both  the source and target host systems. The first modification involves setting the ''xend_relocation_server'' value to ''yes'':

<pre>
(xend-relocation-server yes)
</pre>

Secondly, the ''xend-relocation-hosts-allow'' value must be changed to define the hosts from which relocation requests will be accepted. This can be a list of hostnames or IP addresses including wildcards. An empty value may also be specified to accept connections from any host:

<pre>
(xend-relocation-hosts-allow '')
</pre>