To prevent possible crashes or data corruptions when InnoDB Plugin opens an ib-file set, it checks that it can fully support the file formats in use within the ib-file set. If the system is restarted following a crash, or a “fast shutdown” (i.e., innodb_fast_shutdown is greater than zero), there may be on-disk data structures (such as redo or undo entries, or doublewrite pages) that are in a “too-new” format for the current software. During the recovery process, serious damage can be done to your data files if these data structures are accessed. The startup check of the file format occurs before any recovery process begins, thereby preventing the problems described in Section 11.4, “Possible Problems”.

Beginning with version 1.0.1 of the InnoDB Plugin, the system tablespace records an identifier or tag for the “highest” file format used by any table in any of the tablespaces that is part of the ib-file set. Checks against this file format tag are controlled by the new configuration parameter innodb_file_format_check, which is ON by default.

If the file format tag in the system tablespace is newer or higher than the highest version supported by the particular currently executing software and if innodb_file_format_check is ON, the following error is issued when the server is started:

InnoDB: Error: the system tablespace is in a
file format that this version doesn't support

You can also set innodb_file_format to a file format name. Doing so prevents the InnoDB Plugin from starting if the current software does not support the file format specified. It also sets the “high water mark” to the value you specify. The ability to set innodb_file_format_check will be useful (with future releases of InnoDB) if you manually “downgrade” all of the tables in an ib-file set (as described in Chapter 11, Downgrading from the InnoDB Plugin). You can then rely on the file format check at startup if you subsequently use an older version of InnoDB to access the ib-file set.

In some limited circumstances, you might want to start the server and use an ib-file set that is in a “too new” format (one that is not supported by the software you are using). If you set the configuration parameter innodb_file_format_check to OFF, the InnoDB Plugin opens the database, but issues this warning message in the error log:

InnoDB: Warning: the system tablespace is in a
file format that this version doesn't support

Note that the parameter innodb_file_format_check affects only what happens when a database is opened, not subsequently. Conversely, the parameter innodb_file_format (which enables a specific format) only determines whether or not a new table can be created in the enabled format and has no effect on whether or not a database can be opened.

The file format tag is a “high water mark”, and as such it is increased after the server is started, if a table in a “higher” format is created or an existing table is accessed for read or write (assuming its format is supported). If you access an existing table in a format higher than the format the running software supports, the system tablespace tag is not updated, but table-level compatibility checking applies (and an error is issued), as described in Section 4.4.2, “Table-access File Format Compatibility Checking”. Any time the high water mark is updated, the value of innodb_file_format_check is updated as well, so the command SELECT @@innodb_file_format_check; displays the name of the newest file format known to be used by tables in the currently open ib-file set and supported by the currently executing software.

To best illustrate this behavior, consider the scenario described in Table 4.1, “InnoDB data file compatibility and related InnoDB parameters”. Imagine that some future version of InnoDB supports the “Cheetah” format and that an ib-file set has been used with that version.