InnoDB File Format Primer

Understanding the on-disk format helps when interpreting inno output.

Tablespace Files (.ibd)

Each .ibd file is a tablespace divided into fixed-size pages (default 16,384 bytes). Every page has:

+------------------+  byte 0
| FIL Header       |  38 bytes -- checksum, page number, prev/next, LSN, type, space_id
+------------------+  byte 38
| Page Body        |  varies by page type
|                  |
+------------------+  byte (page_size - 8)
| FIL Trailer      |  8 bytes -- old checksum, LSN low 32 bits
+------------------+

Page 0 is always FSP_HDR with tablespace metadata including the space ID, tablespace size, FSP flags, and extent descriptors.

Page Types

Type	Value	Description
`ALLOCATED`	0	Freshly allocated, type field not yet initialized
`UNDO_LOG`	2	Stores previous values of modified records
`INODE`	3	File segment inode bookkeeping
`TRX_SYS`	7	Transaction system header (system tablespace only)
`FSP_HDR`	8	File space header, always page 0
`XDES`	9	Extent descriptor for 16,384-page blocks
`BLOB`	10	Externally stored column data
`SDI`	17853	Serialized Dictionary Information (MySQL 8.0+)
`INDEX`	17855	B+Tree index node -- table and index data

See the Page Types reference for the full list.

Checksums

Two primary algorithms protect page integrity:

CRC-32C (default since MySQL 5.7.7) -- hardware-accelerated, XOR of two independent CRC-32C values computed over bytes [4..26) and [38..page_size-8).
Legacy InnoDB (MySQL < 5.7.7) -- custom ut_fold_ulint_pair hash with wrapping u32 arithmetic, computed over the same two byte ranges.

MariaDB 10.5+ introduces a third algorithm, full_crc32, which computes a single CRC-32C over [0..page_size-4) and stores the result in the last 4 bytes of the page. See Checksum Algorithms for details.

LSN (Log Sequence Number)

The LSN is a monotonically increasing counter that tracks position in the redo log. Each page records the LSN in two places:

FIL header (8 bytes at offset 16) -- full 64-bit LSN of the last modification
FIL trailer (4 bytes at offset page_size - 4) -- low 32 bits of the same LSN

A mismatch between the header and trailer LSN values indicates a torn page write -- the page was only partially flushed to disk. The inno checksum command reports these as LSN mismatches.

Redo Log Structure

InnoDB redo logs are organized as a sequence of 512-byte blocks:

Block	Purpose
0	File header (group_id, start_lsn, file_no, creator string)
1	Checkpoint 1
2	Reserved
3	Checkpoint 2
4+	Data blocks (14-byte header, up to 494 bytes of log records, 4-byte checksum)

The creator string in block 0 identifies the server version and vendor (e.g., "MySQL 8.0.32", "Percona XtraDB 8.0.35"). See Redo Log Format for the complete specification.

Page Sizes

InnoDB supports five page sizes: 4K, 8K, 16K (default), 32K, and 64K. The page size is encoded in the FSP flags on page 0 and auto-detected by inno. All byte offsets and ranges in this documentation assume the default 16K page size unless stated otherwise.

IDB Utils Documentation