Redo Log Format

InnoDB redo logs record all modifications to tablespace pages, enabling crash recovery. The logs are organized as a sequence of 512-byte blocks.

File Layout

Block	Purpose
0	File header
1	Checkpoint 1
2	Reserved
3	Checkpoint 2
4+	Data blocks containing log records

File Header (Block 0)

Offset	Size	Field	Description
0	4	Group ID	Redo log group identifier
4	8	Start LSN	LSN of the first log record in this file
12	4	File number	Sequence number within the redo log group
16	32	Creator string	Server version and vendor (e.g., "MySQL 8.0.32", "Percona XtraDB 8.0.35")

The creator string is particularly useful for vendor detection -- it identifies whether the redo log was created by MySQL, Percona XtraDB, or MariaDB.

Checkpoint Blocks (Blocks 1 and 3)

Two checkpoint blocks provide redundancy. InnoDB alternates between them, so at least one is always valid after a crash.

Offset	Size	Field	Description
0	8	Checkpoint number	Monotonically increasing checkpoint counter
8	8	LSN	LSN up to which all changes are flushed to tablespace files
16	4	Byte offset	Offset within the log file for this checkpoint
20	4	Log buffer size	Size of the log buffer in bytes
24	8	Archived LSN	LSN up to which logs have been archived (0 if archiving disabled)

The checkpoint with the higher checkpoint number is the most recent. During crash recovery, InnoDB replays all log records from the checkpoint LSN forward.

Data Block Header (14 bytes)

Each data block (block 4 and beyond) starts with a 14-byte header:

Offset	Size	Field	Description
0	4	Block number	Sequential block number; bit 31 is the flush flag (1 = first block in a flush batch)
4	2	Data length	Number of bytes of log data in this block (max 494)
6	2	First record group offset	Offset to the first complete log record group starting in this block
8	4	Checkpoint number	Checkpoint number when this block was written
12	2	Padding	Reserved bytes

Data Block Checksum

Each block ends with a 4-byte CRC-32C checksum:

Computed over bytes [0..508) of the block
Stored at bytes [508..512)

inno log validates block checksums and reports any corruption.

Block Capacity

Each 512-byte block carries at most 494 bytes of log record data:

[header: 14 bytes][log data: up to 494 bytes][checksum: 4 bytes]
 0             13  14                    507  508           511

Log records can span multiple blocks. The data length field indicates how many bytes of the 494-byte payload contain actual log data (the rest is padding).

File Formats

Legacy Format (MySQL < 8.0.30)

Files: ib_logfile0, ib_logfile1 (or more, controlled by innodb_log_files_in_group)
Fixed-size files, pre-allocated at server startup
Location: MySQL data directory root

New Format (MySQL 8.0.30+)

Files: #innodb_redo/#ib_redo* (numbered sequentially)
Dynamic file creation and removal
Location: #innodb_redo/ subdirectory within the data directory
inno log supports both formats

MLOG Record Types

Redo log records use type codes to identify the operation. Common types include:

Type	Value	Description
`MLOG_1BYTE`	1	Write 1 byte to a page
`MLOG_2BYTES`	2	Write 2 bytes to a page
`MLOG_4BYTES`	4	Write 4 bytes to a page
`MLOG_8BYTES`	8	Write 8 bytes to a page
`MLOG_REC_INSERT`	9	Insert a record (non-compact format)
`MLOG_REC_UPDATE_IN_PLACE`	13	Update a record in place
`MLOG_REC_DELETE`	14	Delete a record
`MLOG_PAGE_CREATE`	16	Create a page
`MLOG_UNDO_INSERT`	20	Insert an undo log record
`MLOG_INIT_FILE_PAGE`	24	Initialize a file page
`MLOG_COMP_REC_INSERT`	38	Insert a record (compact format)
`MLOG_COMP_REC_UPDATE_IN_PLACE`	42	Update a record in place (compact format)
`MLOG_COMP_REC_DELETE`	43	Delete a record (compact format)
`MLOG_COMP_PAGE_CREATE`	44	Create a page (compact format)

The MLOG_COMP_* variants are used by the compact row format (MySQL 5.0+), while the non-compact versions correspond to the older redundant row format.

Usage with inno

# Parse a legacy redo log
inno log -f /var/lib/mysql/ib_logfile0

# Parse a new-format redo log
inno log -f /var/lib/mysql/#innodb_redo/#ib_redo10

# Show specific blocks
inno log -f ib_logfile0 -b 0-10

# Skip empty blocks
inno log -f ib_logfile0 --no-empty

# Verbose output with block checksums
inno log -f ib_logfile0 -v

# JSON output
inno log -f ib_logfile0 --json

IDB Utils Documentation