The Fil Header has eight parts, as follows:

--------
  - root -
  --------
       |
  ----------------------
  |                    |
  |                    |
  --------          --------
  - leaf -  <-->    - leaf -
  --------          --------

Everyone has seen a B-tree and knows that the entries in the root page point to the leaf pages. (I indicate those pointers with vertical '|' bars in the drawing.) But sometimes people miss the detail that leaf pages can also point to each other (I indicate those pointers with a horizontal two-way pointer '<-->' in the drawing). This feature allows InnoDB to navigate from leaf to leaf without having to back up to the root level. This is a sophistication which you won't find in the classic B-tree, which is why InnoDB should perhaps be called a B+-tree instead.

The Page Header has 14 parts, as follows:

(Note: I'll clarify what a "heap" is when I discuss the User Records part of the page.)

Some of the Page Header parts require further explanation:

Records which have been freed (due to deletion or migration) are in a one-way linked list. The PAGE_FREE pointer in the page header points to the first record in the list. The "next" pointer in the record header (specifically, in the record's Extra Bytes) points to the next record in the list.

It's useful to know whether inserts are coming in a constantly ascending sequence. That can affect InnoDB's efficiency.

Warning: Like all record pointers, these point not to the beginning of the record but to its Origin (see the earlier discussion of Record Structure).

These variables contain information (space ID, page number, and byte offset) about index node file segments. InnoDB uses the information for allocating new pages. There are two different variables because InnoDB allocates separately for leaf pages and upper-level pages.

"Infimum" and "supremum" are real English words but they are found only in arcane mathematical treatises, and in InnoDB comments. To InnoDB, an infimum is lower than the lowest possible real value (negative infinity) and a supremum is greater than the greatest possible real value (positive infinity). InnoDB sets up an infimum record and a supremum record automatically at page-create time, and never deletes them. They make a useful barrier to navigation so that "get-prev" won't pass the beginning and "get-next" won't pass the end. Also, the infimum record can be a dummy target for temporary record locks.

The InnoDB code comments distinguish between "the infimum and supremum records" and the "user records" (all other kinds).

It's sometimes unclear whether InnoDB considers the infimum and supremum to be part of the header or not. Their size is fixed and their position is fixed, so I guess so.

In the User Records part of a page, you'll find all the records that the user inserted.

There are two ways to navigate through the user records, depending whether you want to think of their organization as an unordered or an ordered list.

An unordered list is often called a "heap". If you make a pile of stones by saying "whichever one I happen to pick up next will go on top" — rather than organizing them according to size and colour — then you end up with a heap. Similarly, InnoDB does not want to insert new rows according to the B-tree's key order (that would involve expensive shifting of large amounts of data), so it inserts new rows right after the end of the existing rows (at the top of the Free Space part) or wherever there's space left by a deleted row.

But by definition the records of a B-tree must be accessible in order by key value, so there is a record pointer in each record (the "next" field in the Extra Bytes) which points to the next record in key order. In other words, the records are a one-way linked list. So InnoDB can access rows in key order when searching.

I think it's clear what the Free Space part of a page is, from the discussion of other parts.

The Page Directory part of a page has a variable number of record pointers. Sometimes the record pointers are called "slots" or "directory slots". Unlike other DBMSs, InnoDB does not have a slot for every record in the page. Instead it keeps a sparse directory. In a fullish page, there will be one slot for every six records.

The slots track the records' logical order (the order by key rather than the order by placement on the heap). Therefore, if the records are 'A''B''F''D' the slots will be (pointer to 'A') (pointer to 'B') (pointer to 'D') (pointer to 'F'). Because the slots are in key order, and each slot has a fixed size, it's easy to do a binary search of the records on the page via the slots.

(Since the Page Directory does not have a slot for every record, binary search can only give a rough position and then InnoDB must follow the "next" record pointers. InnoDB's "sparse slots" policy also accounts for the n_owned field in the Extra Bytes part of a record: n_owned indicates how many more records must be gone through because they don't have their own slots.)

The Fil Trailer has one part, as follows:

The final part of a page, the fil trailer (or File Page Trailer), exists because InnoDB's architect worried about integrity. It's impossible for a page to be only half-written, or corrupted by crashes, because the log-recovery mechanism restores to a consistent state. But if something goes really wrong, then it's nice to have a checksum, and to have a value at the very end of the page which must be the same as a value at the very beginning of the page.