5.4.4 The Binary Log

The binary log contains “events” that describe database changes such as table creation operations or changes to table data. It also contains events for statements that potentially could have made changes (for example, a DELETE which matched no rows), unless row-based logging is used. The binary log also contains information about how long each statement took that updated data. The binary log has two important purposes:

The binary log is not used for statements such as SELECT or SHOW that do not modify data. To log all statements (for example, to identify a problem query), use the general query log. See Section 5.4.3, “The General Query Log”.

Running a server with binary logging enabled makes performance slightly slower. However, the benefits of the binary log in enabling you to set up replication and for restore operations generally outweigh this minor performance decrement.

The binary log is resilient to unexpected halts. Only complete events or transactions are logged or read back.

Passwords in statements written to the binary log are rewritten by the server not to occur literally in plain text. See also Section 6.1.2.3, “Passwords and Logging”.

The following discussion describes some of the server options and variables that affect the operation of binary logging. For a complete list, see Section 17.1.6.4, “Binary Logging Options and Variables”.

Binary logging is enabled by default (the log_bin system variable is set to ON). The exception is if you use mysqld to initialize the data directory manually by invoking it with the --initialize or --initialize-insecure option, when binary logging is disabled by default, but can be enabled by specifying the --log-bin option.

To disable binary logging, you can specify the --skip-log-bin or --disable-log-bin option at startup. If either of these options is specified and --log-bin is also specified, the option specified later takes precedence.

The --log-slave-updates and --slave-preserve-commit-order options require binary logging. If you disable binary logging, either omit these options, or specify --skip-log-slave-updates and --skip-slave-preserve-commit-order. MySQL disables these options by default when --skip-log-bin or --disable-log-bin is specified. If you specify --log-slave-updates or --slave-preserve-commit-order together with --skip-log-bin or --disable-log-bin, a warning or error message is issued.

The --log-bin[=base_name] option is used to specify the base name for binary log files. If you do not supply the --log-bin option, MySQL uses binlog as the default base name for the binary log files. For compatibility with earlier releases, if you supply the --log-bin option with no string or with an empty string, the base name defaults to host_name-bin, using the name of the host machine. It is recommended that you specify a base name, so that if the host name changes, you can easily continue to use the same binary log file names (see Section B.6.7, “Known Issues in MySQL”). If you supply an extension in the log name (for example, --log-bin=base_name.extension), the extension is silently removed and ignored.

mysqld appends a numeric extension to the binary log base name to generate binary log file names. The number increases each time the server creates a new log file, thus creating an ordered series of files. The server creates a new file in the series each time it starts or flushes the logs. The server also creates a new binary log file automatically after the current log's size reaches max_binlog_size. A binary log file may become larger than max_binlog_size if you are using large transactions because a transaction is written to the file in one piece, never split between files.

To keep track of which binary log files have been used, mysqld also creates a binary log index file that contains the names of all used binary log files. By default, this has the same base name as the binary log file, with the extension '.index'. You can change the name of the binary log index file with the --log-bin-index[=file_name] option. You should not manually edit this file while mysqld is running; doing so would confuse mysqld.

The term “binary log file” generally denotes an individual numbered file containing database events. The term “binary log” collectively denotes the set of numbered binary log files plus the index file.

The default location for binary log files and the binary log index file is the data directory. You can use the --log-bin option to specify an alternative location, by adding a leading absolute path name to the base name to specify a different directory. When the server reads an entry from the binary log index file, which tracks the binary log files that have been used, it checks whether the entry contains a relative path. If it does, the relative part of the path is replaced with the absolute path set using the --log-bin option. An absolute path recorded in the binary log index file remains unchanged; in such a case, the index file must be edited manually to enable a new path or paths to be used. The binary log file base name and any specified path are available as the log_bin_basename system variable.

In MySQL 5.7, a server ID had to be specified when binary logging was enabled, or the server would not start. In MySQL 8.0, the server_id system variable is set to 1 by default. The server can be started with this default ID when binary logging is enabled, but an informational message is issued if you do not specify a server ID explicitly using the --server-id option. For servers that are used in a replication topology, you must specify a unique nonzero server ID for each server.

A client that has privileges sufficient to set restricted session system variables (see Section 5.1.9.1, “System Variable Privileges”) can disable binary logging of its own statements by using a SET sql_log_bin=OFF statement.

By default, the server logs the length of the event as well as the event itself and uses this to verify that the event was written correctly. You can also cause the server to write checksums for the events by setting the binlog_checksum system variable. When reading back from the binary log, the master uses the event length by default, but can be made to use checksums if available by enabling the master_verify_checksum system variable. The slave I/O thread also verifies events received from the master. You can cause the slave SQL thread to use checksums if available when reading from the relay log by enabling the slave_sql_verify_checksum system variable.

The format of the events recorded in the binary log is dependent on the binary logging format. Three format types are supported: row-based logging, statement-based logging and mixed-base logging. The binary logging format used depends on the MySQL version. For general descriptions of the logging formats, see Section 5.4.4.1, “Binary Logging Formats”. For detailed information about the format of the binary log, see MySQL Internals: The Binary Log.

The server evaluates the --binlog-do-db and --binlog-ignore-db options in the same way as it does the --replicate-do-db and --replicate-ignore-db options. For information about how this is done, see Section 17.2.5.1, “Evaluation of Database-Level Replication and Binary Logging Options”.

A replication slave server is started with the --log-slave-updates setting enabled by default, meaning that the slave writes to its own binary log any data modifications that are received from the replication master. The binary log must be enabled for this setting to work (see Section 17.1.6.3, “Replication Slave Options and Variables”). This setting enables the slave to act as a master to other slaves in chained replication.

You can delete all binary log files with the RESET MASTER statement, or a subset of them with PURGE BINARY LOGS. See Section 13.7.7.6, “RESET Syntax”, and Section 13.4.1.1, “PURGE BINARY LOGS Syntax”.

If you are using replication, you should not delete old binary log files on the master until you are sure that no slave still needs to use them. For example, if your slaves never run more than three days behind, once a day you can execute mysqladmin flush-logs on the master and then remove any logs that are more than three days old. You can remove the files manually, but it is preferable to use PURGE BINARY LOGS, which also safely updates the binary log index file for you (and which can take a date argument). See Section 13.4.1.1, “PURGE BINARY LOGS Syntax”.

You can display the contents of binary log files with the mysqlbinlog utility. This can be useful when you want to reprocess statements in the log for a recovery operation. For example, you can update a MySQL server from the binary log as follows:

shell> mysqlbinlog log_file | mysql -h server_name

mysqlbinlog also can be used to display replication slave relay log file contents because they are written using the same format as binary log files. For more information on the mysqlbinlog utility and how to use it, see Section 4.6.8, “mysqlbinlog — Utility for Processing Binary Log Files”. For more information about the binary log and recovery operations, see Section 7.5, “Point-in-Time (Incremental) Recovery Using the Binary Log”.

Binary logging is done immediately after a statement or transaction completes but before any locks are released or any commit is done. This ensures that the log is logged in commit order.

Updates to nontransactional tables are stored in the binary log immediately after execution.

Within an uncommitted transaction, all updates (UPDATE, DELETE, or INSERT) that change transactional tables such as InnoDB tables are cached until a COMMIT statement is received by the server. At that point, mysqld writes the entire transaction to the binary log before the COMMIT is executed.

Modifications to nontransactional tables cannot be rolled back. If a transaction that is rolled back includes modifications to nontransactional tables, the entire transaction is logged with a ROLLBACK statement at the end to ensure that the modifications to those tables are replicated.

When a thread that handles the transaction starts, it allocates a buffer of binlog_cache_size to buffer statements. If a statement is bigger than this, the thread opens a temporary file to store the transaction. The temporary file is deleted when the thread ends.

The Binlog_cache_use status variable shows the number of transactions that used this buffer (and possibly a temporary file) for storing statements. The Binlog_cache_disk_use status variable shows how many of those transactions actually had to use a temporary file. These two variables can be used for tuning binlog_cache_size to a large enough value that avoids the use of temporary files.

The max_binlog_cache_size system variable (default 4GB, which is also the maximum) can be used to restrict the total size used to cache a multiple-statement transaction. If a transaction is larger than this many bytes, it fails and rolls back. The minimum value is 4096.

If you are using the binary log and row based logging, concurrent inserts are converted to normal inserts for CREATE ... SELECT or INSERT ... SELECT statements. This is done to ensure that you can re-create an exact copy of your tables by applying the log during a backup operation. If you are using statement-based logging, the original statement is written to the log.

The binary log format has some known limitations that can affect recovery from backups. See Section 17.4.1, “Replication Features and Issues”.

Binary logging for stored programs is done as described in Section 24.7, “Binary Logging of Stored Programs”.

Note that the binary log format differs in MySQL 8.0 from previous versions of MySQL, due to enhancements in replication. See Section 17.4.2, “Replication Compatibility Between MySQL Versions”.

If the server is unable to write to the binary log, flush binary log files, or synchronize the binary log to disk, the binary log on the replication master can become inconsistent and replication slaves can lose synchronization with the master. The binlog_error_action system variable controls the action taken if an error of this type is encountered with the binary log.

By default, the binary log is synchronized to disk at each write (sync_binlog=1). If sync_binlog was not enabled, and the operating system or machine (not only the MySQL server) crashed, there is a chance that the last statements of the binary log could be lost. To prevent this, enable the sync_binlog system variable to synchronize the binary log to disk after every N commit groups. See Section 5.1.8, “Server System Variables”. The safest value for sync_binlog is 1 (the default), but this is also the slowest.

In earlier MySQL releases, there was a chance of inconsistency between the table content and binary log content if a crash occurred, even with sync_binlog set to 1. For example, if you are using InnoDB tables and the MySQL server processes a COMMIT statement, it writes many prepared transactions to the binary log in sequence, synchronizes the binary log, and then commits the transaction into InnoDB. If the server crashed between those two operations, the transaction would be rolled back by InnoDB at restart but still exist in the binary log. Such an issue was resolved in previous releases by enabling InnoDB support for two-phase commit in XA transactions. In 5.8.0 and higher, the InnoDB support for two-phase commit in XA transactions is always enabled.

InnoDB support for two-phase commit in XA transactions ensures that the binary log and InnoDB data files are synchronized. However, the MySQL server should also be configured to synchronize the binary log and the InnoDB logs to disk before committing the transaction. The InnoDB logs are synchronized by default, and sync_binlog=1 ensures the binary log is synchronized. The effect of implicit InnoDB support for two-phase commit in XA transactions and sync_binlog=1 is that at restart after a crash, after doing a rollback of transactions, the MySQL server scans the latest binary log file to collect transaction xid values and calculate the last valid position in the binary log file. The MySQL server then tells InnoDB to complete any prepared transactions that were successfully written to the to the binary log, and truncates the binary log to the last valid position. This ensures that the binary log reflects the exact data of InnoDB tables, and therefore the slave remains in synchrony with the master because it does not receive a statement which has been rolled back.

If the MySQL server discovers at crash recovery that the binary log is shorter than it should have been, it lacks at least one successfully committed InnoDB transaction. This should not happen if sync_binlog=1 and the disk/file system do an actual sync when they are requested to (some do not), so the server prints an error message The binary log file_name is shorter than its expected size. In this case, this binary log is not correct and replication should be restarted from a fresh snapshot of the master's data.

The session values of the following system variables are written to the binary log and honored by the replication slave when parsing the binary log: