InnoDB 1.1 incorporates several checks to guard against the possible crashes and data corruptions that might occur if you run an older release of the MySQL server on InnoDB data files using a newer file format. These checks take place when the server is started, and when you first access a table. This section describes these checks, how you can control them, and error and warning conditions that might arise.
Backward Compatibility
Considerations of backward compatibility only apply when using a recent version of InnoDB (the InnoDB Plugin, or MySQL 5.5 and higher with InnoDB 1.1) alongside an older one (MySQL 5.1 or earlier, with the built-in InnoDB rather than the InnoDB Plugin). To minimize the chance of compatibility issues, you can standardize on the InnoDB Plugin for all your MySQL 5.1 and earlier database servers.
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. InnoDB 1.1 includes a 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 (such as performance improvements and bug fixes), and still preserve the option of using your database with a prior version of InnoDB, by preventing accidental use of new features that create downward-incompatible disk files.
If a version of InnoDB supports a particular file format (whether or not that format is the default), you can query and update any table that requires that format or an earlier format. Only the creation of new tables using new features is limited based on the particular file format enabled. Conversely, if a tablespace contains a table or index that uses a file format that is not supported by the currently running software, it cannot be accessed at all, even for read access.
The only way to “downgrade” an InnoDB tablespace to
an earlier file format is to copy the data to a new table, in a
tablespace that uses the earlier format. This can be done with the
ALTER TABLE statement, as described
in Section 13.7.4.4, “Downgrading the File Format”.
The easiest way to determine the file format of an existing
InnoDB tablespace is to examine the properties of the table it
contains, using the SHOW TABLE STATUS command
or querying the table
INFORMATION_SCHEMA.TABLES. If the
Row_format of the table is reported as
'Compressed' or 'Dynamic',
the tablespace containing the table uses the “Barracuda” format.
Otherwise, it uses the prior InnoDB file format, “Antelope”.
Internal Details
Every InnoDB per-table tablespace (represented by a
*.ibd file) file is labeled with a file format
identifier. The system tablespace (represented by the
ibdata files) is tagged with the
“highest” file format in use in a group of InnoDB
database files, and this tag is checked when the files are opened.
Creating a compressed table, or a table with ROW_FORMAT=DYNAMIC,
updates the file header for the corresponding
.ibd file and the table type in the InnoDB
data dictionary with the identifier for the “Barracuda” file
format. From that point forward, the table cannot be used with a
version of InnoDB that does not support this new file format. To
protect against anomalous behavior, InnoDB version 5.0.21 and
later performs a compatibility check when the table is opened. (In
many cases, the ALTER TABLE
statement recreates a table and thus changes its properties. The
special case of adding or dropping indexes without rebuilding the
table is described in Section 13.7.2, “Fast Index Creation in the InnoDB Storage Engine”.)
Definition of ib-file set
To avoid confusion, for the purposes of this discussion we define the term “ib-file set” to mean the set of operating system files that InnoDB manages as a unit. The ib-file set includes the following files:
The system tablespace (one or more
ibdatafiles) that contain internal system information (including internal catalogs and undo information) and may include user data and indexes.Zero or more single-table tablespaces (also called “file per table” files, named
*.ibdfiles).InnoDB log files; usually two,
ib_logfile0andib_logfile1. Used for crash recovery and in backups.
An “ib-file set” does not include the corresponding
.frm files that contain metadata about InnoDB
tables. The .frm files are created and managed
by MySQL, and can sometimes get out of sync with the internal
metadata in InnoDB.
Multiple tables, even from more than one database, can be stored in a single “ib-file set”. (In MySQL, a “database” is a logical collection of tables, what other systems refer to as a “schema” or “catalog”.)
To prevent possible crashes or data corruptions when InnoDB
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 13.7.11.4, “Possible Problems”.
Beginning with version InnoDB 1.0.1, 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 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 InnoDB 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 Section 13.7.11, “Downgrading the InnoDB Storage Engine”). 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, InnoDB 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
This is a very dangerous setting, as it permits the recovery
process to run, possibly corrupting your database if the
previous shutdown was a crash or “fast shutdown”.
You should only set
innodb_file_format_check
to OFF if you are sure that the previous
shutdown was done with
innodb_fast_shutdown=0, so that essentially
no recovery process occurs. In a future release, this
parameter setting may be renamed from OFF
to UNSAFE. (However, until there are newer
releases of InnoDB that support additional file formats, even
disabling the startup checking is in fact
“safe”.)
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 13.7.4.2.2, “Compatibility Check When a Table Is Opened”.
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 13.8, “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.
Table 13.8. InnoDB Data File Compatibility and Related InnoDB Parameters
| innodb file format check | innodb file format | Highest file format used in ib-file set | Highest file format supported by InnoDB | Result |
|---|---|---|---|---|
OFF | Antelope or Barracuda | Barracuda | Barracuda | Database can be opened; tables can be created which require “Antelope” or “Barracuda” file format |
OFF | Antelope or Barracuda | Cheetah | Barracuda | Database can be opened with a warning, since the database contains files in a “too new” format; tables can be created in “Antelope” or “Barracuda” file format; tables in “Cheetah” format cannot be accessed |
OFF | Cheetah | Barracuda | Barracuda | Database cannot be opened;
innodb_file_format
cannot be set to “Cheetah” |
ON | Antelope or Barracuda | Barracuda | Barracuda | Database can be opened; tables can be created in “Antelope” or “Barracuda” file format |
ON | Antelope or Barracuda | Cheetah | Barracuda | Database cannot be opened, since the database contains files in a “too new” format (“Cheetah”) |
ON | Cheetah | Barracuda | Barracuda | Database cannot be opened;
innodb_file_format
cannot be set to “Cheetah” |
When a table is first accessed, InnoDB (including some releases prior to InnoDB 1.0) checks that the file format of the tablespace in which the table is stored is fully supported. This check prevents crashes or corruptions that would otherwise occur when tables using a “too new” data structure are encountered.
All tables using any file format supported by a release can be
read or written (assuming the user has sufficient privileges).
The setting of the system configuration parameter
innodb_file_format can prevent
creating a new table that uses specific file formats, even if
they are supported by a given release. Such a setting might be
used to preserve backward compatibility, but it does not prevent
accessing any table that uses any supported format.
As noted in Section 13.7.4, “InnoDB File Format Management”, versions of MySQL older than 5.0.21 cannot reliably use database files created by newer versions if a new file format was used when a table was created. To prevent various error conditions or corruptions, InnoDB checks file format compatibility when it opens a file (for example, upon first access to a table). If the currently running version of InnoDB does not support the file format identified by the table type in the InnoDB data dictionary, MySQL reports the following error:
ERROR 1146 (42S02): Table 'test.t1' doesn't exist
InnoDB also writes a message to the error log:
InnoDB: tabletest/t1: unknown table type33
The table type should be equal to the tablespace flags, which contains the file format version as discussed in Section 13.7.4.3, “Identifying the File Format in Use”.
Versions of InnoDB prior to MySQL 4.1 did not include table format identifiers in the database files, and versions prior to MySQL 5.0.21 did not include a table format compatibility check. Therefore, there is no way to ensure proper operations if a table in a “too new” format is used with versions of InnoDB prior to 5.0.21.
The file format management capability in InnoDB 1.0 and higher (tablespace tagging and run-time checks) allows InnoDB to verify as soon as possible that the running version of software can properly process the tables existing in the database.
If you permit InnoDB to open a database containing files in a
format it does not support (by setting the parameter
innodb_file_format_check to
OFF), the table-level checking described in
this section still applies.
Users are strongly urged not to use database files that contain “Barracuda” file format tables with releases of InnoDB older than the MySQL 5.1 with the InnoDB Plugin. It is possible to “downgrade” such tables to the “Antelope” format with the procedure described in Section 13.7.4.4, “Downgrading the File Format”.