database:version:mysql

15.13 Implementation of Multi-Versioning
Because InnoDB is a multi-versioned database, it must keep information about old versions of rows in the tablespace. This information is stored in a data structure called a rollback segment after an analogous data structure in Oracle.
Internally, InnoDB adds two fields to each row stored in the database. A 6-byte field indicates the transaction identifier for the last transaction that inserted or updated the row. Also, a deletion is treated internally as an update where a special bit in the row is set to mark it as deleted. Each row also contains a 7-byte field called the roll pointer. The roll pointer points to an undo log record written to the rollback segment. If the row was updated, the undo log record contains the information necessary to rebuild the content of the row before it was updated.
InnoDB uses the information in the rollback segment to perform the undo operations needed in a transaction rollback. It also uses the information to build earlier versions of a row for a consistent read.

http://www.sourcekeg.co.uk/www.mysql.com/doc/mysql/en/InnoDB_Multi-Versioning.html