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