DataTable truncation

Question

I have a JSF page access a session scoped bean that maintains a datatable. When I first load the page, it correctly displays rows 1 through 20 (the values of the variables configured for "first" and "rows"). There are 150 items in the list ("#{alerts}"). However, when I perform any operation from the page (commandLink pointing to the session bean) the underlying collection is being replaced by a collection that only holds the 20 items that were displayed. Any ideas???
I have the following datatable setup (It's the MyFaces/Tomahawk dataTable):
<t:dataTable id="grid"
             sortColumn="#{alertSession.sortColumn}"
             sortAscending="#{alertSession.sortAscending}"
             preserveDataModel="true"
             preserveSorting="true"
             rows="#{alertFilter.pageSize}"
             first="#{alertSession.firstRow}"
             value="#{alerts}"
             var="alert" >

Answer 1

Followup: if I remove the "preserveDataModel" attribute the problem goes away. Now I confess I don't understand that attribute the way I thought I did. I had thought is was a MyFaces workaround for the fact that JSF pages are loading the data model twice due to the processing performed during the various states of the page loading. However, it seems to be causing the current view of the model (i.e. the 20 rows that I'm displaying in the table) to be put back into the model on the server side, destroying the original collection.