As InnoDB evolves, new on-disk data structures are sometimes required to support new features. This release of InnoDB introduces two such new data structures: compressed tables (see Chapter 3, InnoDB Data Compression), and long variable-length columns stored off-page (see Chapter 5, Storage of Variable-Length Columns). These new data structures are not compatible with prior versions of InnoDB. Note, however, that the other new features of the InnoDB Plugin do not require the use of the new file format.

In general, a newer version of InnoDB may create a table or index that cannot safely be read or written with a prior version of InnoDB without risk of crashes, hangs, wrong results or corruptions. The InnoDB Plugin introduces a new mechanism to guard against these conditions, and to help preserve compatibility among database files and versions of InnoDB. This mechanism lets you take advantage of some new features of an InnoDB release (e.g., performance improvements and bug fixes), and still preserve the option of using your database with a prior version of InnoDB, by precluding the use of new features that create downward incompatible on-disk data structures.