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, 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.
Changing the page size is not a supported operation and there
is no guarantee that InnoDB will
function normally with a page size other than 16KB. Problems
compiling or running InnoDB may occur. In particular,
ROW_FORMAT=COMPRESSED in the Barracuda file
format assumes that the page size is at most 16KB and uses
14-bit pointers.
A version of InnoDB built for one
page size cannot use data files or log files from a version
built for a different page size.