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 14.19.2, “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”.
To change the number or the size of your InnoDB
redo 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, configureinnodb_log_file_size
. To increase the number of log files, configureinnodb_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.
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”.