When a row in a table that does not have a clustering index is updated, and if the modified row no longer fits on the old page, then SQL Server moves the row to a new page and leaves a forwarding pointer in the old page. The forwarding pointer saves the need to modify the non-clustered indexes to make them reflect the row’s new location; however, extra I/O resources are now required for subsequent retrieval of this row using non-clustered indexes.
The set of buffer pages that are currently available for immediate re-use. These are SQL Server memory pages that are not currently allocated to any cache. The Free List is maintained by the Lazy Writer process (see glossary definition of ). If the Free List becomes empty, free pages are not immediately available to SQL Server when required, and the connections needing them might have to wait while SQL Server makes buffers available. This can result in lower performance.