Fragmentation exists when indexes have pages in which the logical ordering, based on the key value, does not match the physical ordering inside the data file. All leaf pages of an index contain pointers to the next and the previous pages in the index. This forms a doubly linked list of all index/data pages. Ideally, the physical order of the pages in the data file should match the logical ordering. Overall disk throughput is increased significantly when the physical ordering matches the logical ordering of the data. This leads to much better performance for certain types of queries. When the physical ordering does not match the logical ordering, disk throughput can become less efficient, because the disk head must move back and forth to gather the index pages instead of scanning forward in one direction. Fragmentation affects I/O performance, but has no effect on performance of queries whose data pages reside in the SQL Server data cache.