The Ultimate Guide to High Availability with MariaDB

Undo Log

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The undo log contains copies of data being modified by active InnoDB transactions. The original data can then be restored, or viewed by a consistent read.

It's not a log file that can be viewed on disk in the usual sense, such as the error log or slow query log, rather an area of storage.

The undo log is usually part of the physical system tablespace, but from MariaDB 10.0, the innodb_undo_directory and innodb_undo_tablespaces system variables can be used to split into different tablespaces and store in a different location.

Each insert or update portion of the undo log is known as a rollback segment. The innodb_undo_logs system variable specifies the number of rollback segments to be used per transaction.

The related innodb_available_undo_logs status variable stores the total number of available InnoDB undo logs.

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