7.4.6 Server Log Maintenance

As described in Section 7.4, “MySQL Server Logs”, MySQL Server can create several different log files to help you see what activity is taking place. However, you must clean up these files regularly to ensure that the logs do not take up too much disk space.

When using MySQL with logging enabled, you may want to back up and remove old log files from time to time and tell MySQL to start logging to new files. See Section 9.2, “Database Backup Methods”.

On a Linux (Red Hat) installation, you can use the mysql-log-rotate script for log maintenance. If you installed MySQL from an RPM distribution, this script should have been installed automatically. Be careful with this script if you are using the binary log for replication. You should not remove binary logs until you are certain that their contents have been processed by all replicas.

On other systems, you must install a short script yourself that you start from cron (or its equivalent) for handling log files.

Binary log files are automatically removed after the server's binary log expiration period. Removal of the files can take place at startup and when the binary log is flushed. The default binary log expiration period is 30 days. To specify an alternative expiration period, use the binlog_expire_logs_seconds system variable. If you are using replication, you should specify an expiration period that is no lower than the maximum amount of time your replicas might lag behind the source. To remove binary logs on demand, use the PURGE BINARY LOGS statement (see Section 15.4.1.1, “PURGE BINARY LOGS Statement”).

To force MySQL to start using new log files, flush the logs. Log flushing occurs when you execute a FLUSH LOGS statement or a mysqladmin flush-logs, mysqladmin refresh, mysqldump --flush-logs, or mysqldump --source-data command. See Section 15.7.8.3, “FLUSH Statement”, Section 6.5.2, “mysqladmin — A MySQL Server Administration Program”, and Section 6.5.4, “mysqldump — A Database Backup Program”. In addition, the server flushes the binary log automatically when current binary log file size reaches the value of the max_binlog_size system variable.

FLUSH LOGS supports optional modifiers to enable selective flushing of individual logs (for example, FLUSH BINARY LOGS). See Section 15.7.8.3, “FLUSH Statement”.

A log-flushing operation has the following effects:

• If binary logging is enabled, the server closes the current binary log file and opens a new log file with the next sequence number.

• If general query logging or slow query logging to a log file is enabled, the server closes and reopens the log file.

• If the server was started with the --log-error option to cause the error log to be written to a file, the server closes and reopens the log file.

Execution of log-flushing statements or commands requires connecting to the server using an account that has the RELOAD privilege. On Unix and Unix-like systems, another way to flush the logs is to send a signal to the server, which can be done by root or the account that owns the server process. (See Section 6.10, “Unix Signal Handling in MySQL”.) Signals enable log flushing to be performed without having to connect to the server:

• A SIGHUP signal flushes all the logs. However, SIGHUP has additional effects other than log flushing that might be undesirable.

• SIGUSR1 causes the server to flush the error log, general query log, and slow query log. If you are interested in flushing only those logs, SIGUSR1 can be used as a more “lightweight” signal that does not have the SIGHUP effects that are unrelated to logs.

As mentioned previously, flushing the binary log creates a new binary log file, whereas flushing the general query log, slow query log, or error log just closes and reopens the log file. For the latter logs, to cause a new log file to be created on Unix, rename the current log file first before flushing it. At flush time, the server opens the new log file with the original name. For example, if the general query log, slow query log, and error log files are named mysql.log, mysql-slow.log, and err.log, you can use a series of commands like this from the command line:

cd mysql-data-directory
mv mysql.log mysql.log.old
mv mysql-slow.log mysql-slow.log.old
mv err.log err.log.old
mysqladmin flush-logs

On Windows, use rename rather than mv.

At this point, you can make a backup of mysql.log.old, mysql-slow.log.old, and err.log.old, then remove them from disk.

To rename the general query log or slow query log at runtime, first connect to the server and disable the log:

SET GLOBAL general_log = 'OFF';
SET GLOBAL slow_query_log = 'OFF';

With the logs disabled, rename the log files externally (for example, from the command line). Then enable the logs again:

SET GLOBAL general_log = 'ON';
SET GLOBAL slow_query_log = 'ON';

This method works on any platform and does not require a server restart.

Note

For the server to recreate a given log file after you have renamed the file externally, the file location must be writable by the server. This may not always be the case. For example, on Linux, the server might write the error log as /var/log/mysqld.log, where /var/log is owned by root and not writable by mysqld. In this case, log-flushing operations fail to create a new log file.

To handle this situation, you must manually create the new log file with the proper ownership after renaming the original log file. For example, execute these commands as root:

mv /var/log/mysqld.log /var/log/mysqld.log.old
install -omysql -gmysql -m0644 /dev/null /var/log/mysqld.log