15.6.6 Undo Logs

An undo log is a collection of undo log records associated with a single transaction. An undo log record contains information about how to undo the latest change by a transaction to a clustered index record. If another transaction needs to see the original data (as part of a consistent read operation), the unmodified data is retrieved from undo log records. Undo logs exist within undo log segments, which are contained within rollback segments. Rollback segments reside in undo tablespaces and in the global temporary tablespace. For more information about undo tablespaces, see Section 15.6.3.4, “Undo Tablespaces”. For information about multi-versioning, see Section 15.3, “InnoDB Multi-Versioning”.

The global temporary tablespace (ibtmp1) and each undo tablespace individually support a maximum of 128 rollback segments. The innodb_rollback_segments variable defines the number of rollback segments. Each rollback segment supports up to 1023 concurrent data-modifying transactions. See Configuring the Number of Rollback Segments.