Note: this article was originally published on http://blogs.innodb.com on July 25, 2011 by Sunny Bains.
The InnoDB UNDO entries reside in a special system table called the UNDO log. This log is made up of several segments. These segments are called rollback segments. A segment in InnoDB is similar to what a file would be in a file system,e.g., user tables and indexes are also stored as separate segments within the same tablespace, only their format is different. In that sense there is nothing special about InnoDB UNDO logs. This feature allows storing of the UNDO log across several tablespaces.
UNDO logs contain the before image of modified records. There are two types of UNDO records, one for insert and another for updates. The insert UNDO records can be …[Read more]