MySQL’s undo log

Short Braindump:
– undo log == rollback segment
– everything kept within the tablespace
– primarily for removing data changes that had been written to disk when a crash occurred, but should not have been written, because they were for uncommitted transactions.”
– “History list length 6 is number of unpurged transactions in undo space. It is increased as transactions which have done updates are commited and decreased as purge runs. ” – comes from http://www.mysqlperformanceblog.com/2006/07/17/show-innodb-status-walk-through/
– number of undo log entries for a specific transaction can also be found in the TRANSACTION section of INNODB STATUS

References:
1) http://forge.mysql.com/wiki/MySQL_Internals_Files_In_InnoDB_Sources – “Some Notes About Structures”
2) http://dev.mysql.com/doc/refman/5.1/en/innodb-multi-versioning.html
3) http://dev.mysql.com/doc/refman/5.5/en/innodb-multiple-rollback-segments.html