the fillfactor effect specifies the percentage of space filled on index data pages when an index is initially created. The default fillfactor setting of zero, which can be altered at the global server level through a configuration option, will cause an index to be almost filled to capacity, with only a small amount of space being left at the upper level region of the index. A 100% setting completely fills each index page.
One important thing to remember is that this amount is not adhered to after the index is first built. An index can be rebuilt and the original fillfactor setting re-instituted with the variety of DBCC index rebuild or ALTER INDEX (SQL Server 2005) commands.
So, what exactly are the considerations with fillfactor? Higher fillfactor settings should result in less index pages which in turn should result in fewer pages read during scan operations. As been mentioned many times already, less I/O generally equates to better performance.
However, high fillfactor settings can also result in page splits for clustered indexes when SQL Server enforces the sort order of the clustered index during INSERT or UPDATE actions. This happens because SQL Server does not have room on an index page for the requested change, so it has to split the page to perform the modification. This can result in performance degradation and can be confirmed by carefully watching the page splits counter, available from the page_splits query below:
counter_name = 'Page Splits/sec' and
object_name like '%Access methods%'
If there are tables present that are primarily read only, a fillfactor setting of 100 should be used to reduce the number of produced index pages.
If, however, there are tables present with high rates of INSERT, UPDATE and DELETE activity, lower fillfactor settings of 50-60% should be used. This will need to be coupled with periodic index rebuilds that will re-establish the fillfactor setting to keep DML running smooth through the indexes.
A mixed environment can work well with fillfactor settings in the neighborhood of 75%. One last piece of advice: for small indexes that have few pages, time should not be wasted worrying about fillfactor as it will not be capable of impacting the database’s performance for the worse.