22.2.1.1 Fil Header

The Fil Header has eight parts, as follows:

Name	Size	Remarks
FIL_PAGE_SPACE	4	4 ID of the space the page is in
FIL_PAGE_OFFSET	4	ordinal page number from start of space
FIL_PAGE_PREV	4	offset of previous page in key order
FIL_PAGE_NEXT	4	offset of next page in key order
FIL_PAGE_LSN	8	log serial number of page's latest log record
FIL_PAGE_TYPE	2	current defined types are: FIL_PAGE_INDEX, FIL_PAGE_UNDO_LOG, FIL_PAGE_INODE, FIL_PAGE_IBUF_FREE_LIST
FIL_PAGE_FILE_FLUSH_LSN	8	"the file has been flushed to disk at least up to this lsn" (log serial number), valid only on the first page of the file
FIL_PAGE_ARCH_LOG_NO	4	the latest archived log file number at the time that FIL_PAGE_FILE_FLUSH_LSN was written (in the log)

• FIL_PAGE_SPACE is a necessary identifier because different pages might belong to different (table) spaces within the same file. The word "space" is generic jargon for either "log" or "tablespace".

• FIL_PAGE_PREV and FIL_PAGE_NEXT are the page's "backward" and "forward" pointers. To show what they're about, I'll draw a two-level B-tree.

--------
  - 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 fields FIL_PAGE_FILE_FLUSH_LSN, FIL_PAGE_PREV, and FIL_PAGE_NEXT all have to do with logs, so I'll refer you to my article “How Logs Work With MySQL And InnoDB” on devarticles.com.

• FIL_PAGE_FILE_FLUSH_LSN and FIL_PAGE_ARCH_LOG_NO are valid only for the first page of a data file.