The index page cluster ratio measures the packing and sequencing of index leaf pages on extents for nonclustered indexes and clustered indexes on data-only-locked tables. For queries that need to read more than one leaf page, the leaf level of the index is scanned using next-page or previous-page pointers. If many leaf rows need to be read, 16K I/O can be used on the leaf pages to read one extent at a time. The index page cluster ratio measures fragmentation of the page chain for the leaf level of the index.
