In addition to storing metadata about database objects in the data dictionary, MySQL stores it in serialized form. This data is referred to as serialized dictionary information (SDI). InnoDB stores SDI data within its tablespace files. NDBCLUSTER stores SDI data in the NDB dictionary. Other storage engines store SDI data in .sdi files that are created for a given table in the table's database directory. SDI data is generated in a compact JSON format.

Serialized dictionary information (SDI) is present in all InnoDB tablespace files except for temporary tablespace and undo tablespace files. SDI records in an InnoDB tablespace file only describe table and tablespace objects contained within the tablespace.

SDI data is updated by DDL operations on a table or CHECK TABLE FOR UPGRADE. SDI data is not updated when the MySQL server is upgraded to a new release or version.

The presence of SDI data provides metadata redundancy. For example, if the data dictionary becomes unavailable, object metadata can be extracted directly from InnoDB tablespace files using the ibd2sdi tool.

For InnoDB, an SDI record requires a single index page, which is 16KB in size by default. However, SDI data is compressed to reduce the storage footprint.

For partitioned InnoDB tables comprised of multiple tablespaces, SDI data is stored in the tablespace file of the first partition.

The MySQL server uses an internal API that is accessed during DDL operations to create and maintain SDI records.

The IMPORT TABLE statement imports MyISAM tables based on information contained in .sdi files. For more information, see Section 15.2.6, “IMPORT TABLE Statement”.