The binary log contains all statements that update data or (starting from MySQL 4.1.3) potentially could have updated it (for example, a DELETE which matched no rows). Statements are stored in the form of “events” that describe the modifications. The binary log also contains information about how long each statement took that updated data. The binary log has two important purposes:

For information about server options and variables affecting the operation of binary logging, see Section 14.8.4, “Binary Log Options and Variables”.

The binary log is not used for statements such as SELECT or SHOW that do not modify data. If you want to log all statements (for example, to identify a problem query), use the general query log. See Section 5.3.2, “The General Query Log”.

MySQL Enterprise.  The binary log can also be used to track significant DDL events. Analyzing the binary log in this way is an integral part of the MySQL Enterprise Monitor. For more information, see http://www.mysql.com/products/enterprise/advisors.html.

Running the server with the binary log enabled makes performance about 1% slower. However, the benefits of the binary log for restore operations and in allowing you to set up replication generally outweigh this minor performance decrement.

When started with the --log-bin[=base_name] option, mysqld writes a log file containing all SQL statements that update data (both DDL and DML statements). If no base_name value is given, the default name is the value of the pid-file option (which by default is the name of host machine) followed by -bin. If the basename is given, but not as an absolute pathname, the server writes the file in the data directory. It is recommended that you specify a basename; see Section A.1.8.4, “Open Issues in MySQL”, for the reason.

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 basename to generate binary log filenames. 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 when 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 basename 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.

You can delete all binary log files with the RESET MASTER statement, or a subset of them with PURGE BINARY LOGS. See Section 12.5.6.5, “RESET Syntax”, and Section 12.6.1.1, “PURGE BINARY LOGS Syntax”.

Before MySQL 4.1.9, writes to a binary log file or binary log index file that failed due to a full disk or an exceeded quota resulted in corruption of the file. Starting from MySQL 4.1.9, writes to the binary log file and binary log index file are handled the same way as writes to MyISAM tables. See Section A.1.4.3, “How MySQL Handles a Full Disk”.

The binary log format has some known limitations that can affect recovery from backups, especially in old versions. These caveats, which also affect replication, are listed at Section 14.7, “Replication Features and Known Problems”. One caveat which does not affect replication but only recovery with mysqlbinlog: before MySQL 4.1, mysqlbinlog could not prepare output suitable for mysql if the binary log contained interlaced statements originating from different clients that used temporary tables of the same name. This is fixed in MySQL 4.1. However, the problem still existed for LOAD DATA INFILE statements until it was fixed in MySQL 4.1.8.

A replication slave server by default does not write to its own binary log any data modifications that are received from the replication master. To log these modifications, start the slave with the --log-slave-updates option (see also Section 14.8.3, “Replication Slave Options and Variables”).

Evaluation of update selection options.  The server evaluates the options for logging or ignoring updates to the binary log according to the following rules:

For example, a slave running with only --binlog-do-db=sales does not write to the binary log any statement for which the default database is different from sales (in other words, --binlog-do-db can sometimes mean “ignore other databases”).

If you are using replication, you should not delete old binary log files 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 as of MySQL 4.1). See Section 12.6.1.1, “PURGE BINARY LOGS Syntax”.

A client that has the SUPER privilege can disable binary logging of its own statements by using a SET sql_log_bin=0 statement. See Section 5.1.4, “Session System Variables”.

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 example, you can update a MySQL server from the binary log as follows:

shell> mysqlbinlog log_file | mysql -h server_name

See Section 4.6.6, “mysqlbinlog — Utility for Processing Binary Log Files”, for more information on the mysqlbinlog utility and how to use it. mysqlbinlog also can be used with relay log files because they are written using the same format as binary log files.

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

Updates to non-transactional 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 BDB or 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. When the 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.

Modifications to non-transactional tables cannot be rolled back. If a transaction that is rolled back includes modifications to non-transactional tables, the entire transaction is logged with a ROLLBACK statement at the end to ensure that the modifications to those tables are replicated. This is true as of MySQL 4.0.15.

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 update log or binary log, 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.

The binary log format differs between versions 3.23 and 4.0. (These format changes were required to implement enhancements to replication.) However, MySQL 4.1 has the same binary log format as 4.0. See Section 14.5, “Replication Compatibility Between MySQL Versions”.

By default, the binary log is not synchronized to disk at each write. So if the operating system or machine (not only the MySQL server) crashes, there is a chance that the last statements of the binary log are lost. To prevent this, you can make the binary log be synchronized to disk after every N writes to the binary log, with the sync_binlog system variable. See Section 5.1.3, “System Variables”. 1 is the safest value for sync_binlog, but also the slowest. Even with sync_binlog set to 1, there is still the chance of an inconsistency between the table content and binary log content in case of a crash. For example, if you are using InnoDB tables and the MySQL server processes a COMMIT statement, it writes the whole transaction to the binary log and then commits this transaction into InnoDB. If the server crashes between those two operations, the transaction is rolled back by InnoDB at restart but still exists in the binary log. This problem can be solved with the --innodb-safe-binlog option (available starting from MySQL 4.1.3), which adds consistency between the content of InnoDB tables and the binary log.

For this option to provide a greater degree of safety, the MySQL server should also be configured to synchronize the binary log and the InnoDB logs to disk at every transaction. The InnoDB logs are synchronized by default, and sync_binlog=1 can be used to synchronize the binary log. The effect of this option is that at restart after a crash, after doing a rollback of transactions, the MySQL server cuts rolled back InnoDB transactions from the binary log. This ensures that the binary log reflects the exact data of InnoDB tables, and so, that the slave remains in synchrony with the master (not receiving a statement which has been rolled back).

Note that --innodb-safe-binlog can be used even if the MySQL server updates other storage engines than InnoDB. Only statements and transactions that affect InnoDB tables are subject to removal from the binary log at InnoDB's crash recovery. 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/filesystem do an actual sync when they are requested to (some don't), so the server prints an error message The binary log <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.