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MySQL 5.6 Reference Manual  /  ...  /  The Physical Structure of an InnoDB Index

14.6.2.2 The Physical Structure of an InnoDB Index

All InnoDB indexes are B-trees where the index records are stored in the leaf pages of the tree. The default size of an index page is 16KB.

When new records are inserted into an InnoDB clustered index, InnoDB tries to leave 1/16 of the page free for future insertions and updates of the index records. If index records are inserted in a sequential order (ascending or descending), the resulting index pages are about 15/16 full. If records are inserted in a random order, the pages are from 1/2 to 15/16 full. If the fill factor of an index page drops below 1/2, InnoDB tries to contract the index tree to free the page.

You can define the page size for all InnoDB tablespaces in a MySQL instance by setting the innodb_page_size configuration option prior to initializing the MySQL instance. Once the page size for an instance is defined, you cannot change it without reinitializing the instance. Supported sizes are 16KB, 8KB, and 4KB.

A MySQL instance using a particular InnoDB page size cannot use data files or log files from an instance that uses a different page size.