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14.5.5 InnoDB Rollback Segments

A rollback segment is a storage area that holds copies of data modified by active transactions. If another transaction needs to see the original data (as part of a consistent read operation), the unmodified data is retrieved from this storage area, which is physically part of the system tablespace. For more information about rollback segments and multi-versioning, see Section 14.5.3, “InnoDB Multi-Versioning”.

Prior to MySQL 5.5.4, InnoDB supported a single rollback segment which supported a maximum of 1023 concurrent data-modifying transactions (read-only transactions do not count against the maximum limit). In MySQL 5.5.4 the single rollback segment was divided into 128 rollback segments, each supporting up to 1023 concurrent data-modifying transactions, creating a new limit of approximately 128K concurrent data-modifying transactions. The innodb_rollback_segments option defines how many of the rollback segments in the system tablespace are used for InnoDB transactions.

Each transaction is assigned to one of the rollback segments, and remains tied to that rollback segment for the duration. The increased limit for concurrent data-modifying transactions improves both scalability (higher number of concurrent transactions) and performance (less contention when different transactions access the rollback segments).

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