You can specify the scope of the persistence in addition to persistence frequency on the same page in the Admin Console where you specify persistence frequency, Configurations > config-name > Availability Service (Web Container Availability).
For detailed description of different persistence scopes, see Chapter 7, Configuring High Availability Session Persistence and Failover, in Sun GlassFish Enterprise Server 2.1 High Availability Administration Guide.
With the session persistence scope, the server writes the entire session data to HADB—regardless of whether it has been modified. This mode ensures that the session data in the backend store is always current, but it degrades performance, since all the session data is persisted for every request.
With the modified-session persistence scope, the server examines the state of the HTTP session. If and only if the data has been modified, the server saves the session data to HADB. This mode yields better performance than session mode, because calls to HADB to persist data occur only when the session is modified.
With the modified-attribute persistence scope, there are no cross-references for the attributes, and the application uses setAttribute() and getAttribute() to manipulate HTTP session data. Applications written this way can take advantage of this session scope behavior to obtain better performance.