iDisplayLength always -1

I have a data table using Scroller with server-side processing and the following init:

oTable= jQuery('#request-grid').dataTable({
                "bProcessing": true,
                "bServerSide": true,
                "bPaginate": false,
                "bFilter": false,
                "bScrollCollapse": false,
                "sScrollY": "100px",
                "bDeferRender": true,
                "sScrollX": "100%",
                "sAjaxSource": '/ajax/requests-db.php',
                "sDom":'<"grid-top"lfrip<"clear">>tS<"grid-bottom">',
                "oScroller": {
                        "loadingIndicator": true,
                        "trace": true
                },
                "fnRowCallback": function ( nRow, aData, iDisplayIndex ) {
                    row_css = status_to_class (parseInt(aData[7]));
                    if (row_css !== false ) jQuery(nRow).addClass(row_css);
                },
                "fnInitComplete" : function(oSettings, json) {
                    adjust_table_height();
                    console.log("oTable initialized " + oTable.fnSettings().oScroll.sY);
                },
                "fnCreatedRow": function( nRow, aData, iDataIndex ) {
                    jQuery(nRow).children("td").wrapInner("<div />")
                }
                
            });

I have a strange problem: in the ajax query to the server iDisplayLength value always -1
sEcho:15
iColumns:7
sColumns:
iDisplayStart:0
iDisplayLength:-1
mDataProp_0:0
mDataProp_1:1
mDataProp_2:2
mDataProp_3:3
mDataProp_4:4
mDataProp_5:5
mDataProp_6:6
iSortCol_0:0
sSortDir_0:asc
iSortingCols:1
bSortable_0:true
bSortable_1:true
bSortable_2:true
bSortable_3:true
bSortable_4:true
bSortable_5:true
bSortable_6:true

and my servers-side processing script return all records. This causes a problem when a large number of rows in the table.
Also, during scrolling, the table not make ajax query for additional data-loading like this scroller example: http://datatables.net/release-datatables/extras/Scroller/server-side_processing.html
Who can help solve it?
my table debug data http://debug.datatables.net/exuxon