The auto_failback option determines whether a resource will automatically fail back to its "primary" node, or remain on whatever node is serving it until that node fails, or an administrator intervenes.
The possible values for auto_failback are:
on - enable automatic failbacks
off - disable automatic failbacks
legacy - enable automatic failbacks in systems where all nodes in the cluster do not yet support the auto_failback option.
Both the auto_failback on and off are backwards compatible with the old "nice_failback on" setting.
The default value for auto_failback is "legacy", which will issue a warning at startup. So, make sure you put an auto_failback directive in your ha.cf file (note: auto_failback can be any HeartbeatBoolean value or legacy). Typically, you want to set auto_failback on for an ActiveActive cluster, and commonly to off for an ActivePassive cluster.
NOTE: auto_failback does not have any effect on a Release 2 CRM-style cluster (one configured with crm on). For CRM-style clusters, this has been replaced with the default_resource_stickiness attribute in the CIB.