14.6.6 Redo Log

The redo log is a disk-based data structure used during crash recovery to correct data written by incomplete transactions. During normal operations, the redo log encodes requests to change table data that result from SQL statements or low-level API calls. Modifications that did not finish updating the data files before an unexpected shutdown are replayed automatically during initialization, and before the connections are accepted. For information about the role of the redo log in crash recovery, see Section 14.18.2, “InnoDB Recovery”.

By default, the redo log is physically represented on disk by two files named ib_logfile0 and ib_logfile1. MySQL writes to the redo log files in a circular fashion. Data in the redo log is encoded in terms of records affected; this data is collectively referred to as redo. The passage of data through the redo log is represented by an ever-increasing LSN value.

For related information, see Redo Log File Configuration, and Section 8.5.4, “Optimizing InnoDB Redo Logging”.

Changing the Number or Size of Redo Log Files

To change the number or size of InnoDB redo log files in MySQL 5.6.7 or earlier, perform the following steps:

If innodb_fast_shutdown is set to 2, set innodb_fast_shutdown to 1:
```
mysql> SET GLOBAL innodb_fast_shutdown = 1;
```
After ensuring that innodb_fast_shutdown is not set to 2, stop the MySQL server and make sure that it shuts down without errors (to ensure that there is no information for outstanding transactions in the log).
Copy the old log files into a safe place in case something went wrong during the shutdown and you need them to recover the tablespace.
Delete the old log files from the log file directory.
Edit my.cnf to change the log file configuration.
Start the MySQL server again. mysqld sees that no InnoDB log files exist at startup and creates new ones.

The innodb_fast_shutdown setting is no longer relevant when changing the number or the size of InnoDB log files. Additionally, you are no longer required to remove old log files, although you may still want to copy the old log files to a safe place, as a backup. To change the number or size of InnoDB log files, perform the following steps:

Stop the MySQL server and make sure that it shuts down without errors.
Edit my.cnf to change the log file configuration. To change the log file size, configure innodb_log_file_size. To increase the number of log files, configure innodb_log_files_in_group.
Start the MySQL server again.

If InnoDB detects that the innodb_log_file_size differs from the redo log file size, it writes a log checkpoint, closes and removes the old log files, creates new log files at the requested size, and opens the new log files.

Group Commit for Redo Log Flushing

InnoDB, like any other ACID-compliant database engine, flushes the redo log of a transaction before it is committed. InnoDB uses group commit functionality to group multiple such flush requests together to avoid one flush for each commit. With group commit, InnoDB issues a single write to the log file to perform the commit action for multiple user transactions that commit at about the same time, significantly improving throughput.

For more information about performance of COMMIT and other transactional operations, see Section 8.5.2, “Optimizing InnoDB Transaction Management”.