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InnoDB provides MySQL with a transaction-safe (ACID compliant) storage engine with commit, rollback, and crash recovery capabilities. InnoDB does locking on row level and also provides an Oracle-style consistent non-locking read in SELECT statements. These features increase multiuser concurrency and performance. There is no need for lock escalation in InnoDB, because row level locks in InnoDB fit in very small space. InnoDB is the first storage manager in MySQL to support FOREIGN KEY constraints.
InnoDB has been designed for maximum performance when processing large data volumes. Its CPU efficiency is probably not matched by any other disk-based relational database engine.
InnoDB is used in production at numerous large database sites requiring high performance. The famous Internet news site Slashdot.org runs on InnoDB. Mytrix, Inc. stores over 1 TB of data in InnoDB, and another site handles an average load of 800 inserts/updates per second in InnoDB.
Technically, InnoDB is a complete database backend placed under MySQL. InnoDB has its own buffer pool for caching data and indexes in main memory. InnoDB stores its tables and indexes in a tablespace, which may consist of several files (or raw disk partitions). This is different from, for example, MyISAM tables where each table is stored as a separate file. InnoDB tables can be of any size even on operating systems where file-size is limited to 2 GB.
You can find the latest information about InnoDB at http://www.innodb.com/. The most up-to-date version of the InnoDB manual is always placed there.
InnoDB is published under the same GNU GPL License Version 2 (of June 1991) as MySQL. If you distribute MySQL/InnoDB, and your application does not satisfy the restrictions of the GPL license, you have to buy a commercial MySQL Pro license from https://order.mysql.com/?sub=pg&pg_no=1.