An undo tablespace comprises one or more files that contain undo logs. Undo logs exist within undo log segments, which are contained within rollback segments. By default, rollback segments are physically part of the system tablespace. However, rollback segments can reside in separate undo tablespaces. An undo tablespace is created when the undo log is separated from the system tablespace using the innodb_undo_tablespaces and innodb_undo_directory configuration options.

For more information, see Section 14.7.7, “Storing InnoDB Undo Logs in Separate Tablespaces”.