Large I/Os can now be used to access index pages along with data pages. More accurate costing of large I/Os using cluster ratios was added in 11.9.X. These new statistics values for tables and indexes are stored in systabstats.
Cluster ratios are used to determine if large I/O access will be efficient. The lower the cluster ratio, the less likely a large I/O access will be useful. Look under the heading “Derived statistics” in optdiag or trace flag 302 output (Table Information Block and the Index Selection Block) for the cluster ratios.