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InnoDB 1.1 for MySQL 5.5 User's Guide
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7.10. Group Commit

InnoDB, like any other ACID-compliant database engine, flushes the redo log of a transaction before it is committed. Historically, InnoDB used 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.

Group commit in InnoDB worked until MySQL 4.x, and works once again with MySQL 5.1 with the InnoDB Plugin, and MySQL 5.5 and higher. The introduction of support for the distributed transactions and Two Phase Commit (2PC) in MySQL 5.0 interfered with the InnoDB group commit functionality. This issue is now resolved.

The group commit functionality inside InnoDB works with the Two Phase Commit protocol in MySQL. Re-enabling of the group commit functionality fully ensures that the ordering of commit in the MySQL binlog and the InnoDB logfile is the same as it was before. It means it is totally safe to use the MySQL Enterprise Backup product with InnoDB 1.0.4 (that is, the InnoDB Plugin with MySQL 5.1) and above. When the binlog is enabled, you typically also set the configuration option sync_binlog=0, because group commit for the binary log is only supported if it is set to 0.

Group commit is transparent; you do not need to do anything to take advantage of this significant performance improvement.

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

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