Table of Contents
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 features both require use of the new Barracuda file format.
These new data structures are not compatible with prior versions of InnoDB. The other new features of the InnoDB Plugin are compatible with the original Antelope file format and do not require the Barracuda 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.
The InnoDB Plugin introduces the idea of a named file format and a configuration parameter to enable the use of features that require use of that format. The new file format is the Barracuda format, and the file format supported by prior releases of InnoDB is known as Antelope. Compressed tables and the new row format that stores long columns “off-page” require the use of the Barracuda file format or newer. Future versions of InnoDB may introduce a series of file formats, identified with the names of animals, in ascending alphabetic order.
Beginning with this release, every InnoDB per-table tablespace
file is labeled with a file format identifier. This does not apply
to the system tablespace (the ibdata files) but
only the files of separate tablespaces (the
*.ibd files where tables and indexes are stored
in their own tablespace). As noted below, however, the system
tablespace 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.
In this release, when you create a compressed table, or a table
with ROW_FORMAT=DYNAMIC, the file header for the corresponding
.ibd file and the table type in the InnoDB
data dictionary are updated 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, as described below. (Note that the
ALTER TABLE command in many cases, causes a table to be
recreated and thereby change its properties. The special case of
adding or dropping indexes without rebuilding the table is
described in Chapter 2, Fast Index Creation in the InnoDB Storage Engine.)
If a version of InnoDB supports a particular file format (whether or not it is enabled), you can access and even 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 command, as described in
Section 4.6, “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”.
The new configuration parameter innodb_file_format controls
whether such commands as CREATE TABLE and ALTER TABLE can be
used to create tables that depend on support for the “Barracuda”
file format.
The file format is a dynamic, global parameter that can be
specified in the MySQL option file (my.cnf or
my.ini) or changed with the SET
GLOBAL command, as described in
Section 9.5, “Configuring the InnoDB Plugin”.
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 ibdata
files) 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 *.ibd
files).
(Usually two) InnoDB log files
(ib_logfile0 and
ib_logfile1), used for crash recovery and
in backups.
This collection of files is transactionally consistent, and
recoverable as a unit. An “ib-file set” specifically
does not include the related MySQL .frm files
that contain metadata about InnoDB tables. The
.frm files are created and managed exclusively
by MySQL, and can sometimes get out of sync with the internal
metadata in InnoDB.
Instead of “ib-file set”, we might call such a collection a “database”. However, MySQL uses the word “database” to mean a logical collection of tables, what other systems term a “schema” or “catalog”. Given MySQL terminology, multiple tables (even from more than one database) can be stored in a single “ib-file set”.
The InnoDB Plugin incorporates several checks to guard against the possible crashes and data corruptions that might occur if you use an ib-file set in a file format that is not supported by the software release in use. 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 may arise.
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
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”.)
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.
Table 4.1. 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 which require “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 which require “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 Plugin 1.0) check 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.
Note that 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 4.2, “Named File Formats”, versions of InnoDB 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 (e.g., 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
Furthermore, InnoDB 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 4.5, “Identifying the File Format in Use”.
Versions of InnoDB prior to 4.1 did not include table format identifiers in the database files, and versions prior to 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 new file format management capability introduced with the InnoDB Plugin (comprising 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 InnoDB Plugin. It is possible to “downgrade” such tables to the “Antelope” format (that the built-in InnoDB in MySQL up to version 5.1 supports) with the procedure described in Section 4.6, “Downgrading the File Format”.
Although you may have enabled a given innodb_file_format at a
particular time, unless you create a new table, the database file
format is unchanged. If you do create a new table, the tablespace
containing the table is tagged with the “earliest” or
“simplest” file format that is required for the
table’s features. For example, if you enable file format
“Barracuda”, and create a new table that is not compressed and
does not use ROW_FORMAT=DYNAMIC, the new tablespace that
contains the table is tagged as using file format “Antelope”.
It is easy to identify the file format used by a given tablespace
or table. The table uses the “Barracuda” format if the
Row_format reported by SHOW CREATE
TABLE or INFORMATION_SCHEMA.TABLES is
one of 'Compressed' or
'Dynamic'. (Please note that the
Row_format is a separate column, and ignore the
contents of the Create_options column, which
may contain the string ROW_FORMAT.) If the table in a tablespace
uses neither of those features, the file uses the format supported
by prior releases of InnoDB, now called file format
“Antelope”. Then, the Row_format is one of
'Redundant' or 'Compact'.
The file format identifier is written as part of the tablespace
flags (a 32-bit number) in the *.ibd file in
the 4 bytes starting at position 54 of the file, most significant
byte first. (The first byte of the file is byte zero.) On some
systems, you can display these bytes in hexadecimal with the
command od -t x1 -j 54 -N 4
. If all bytes
are zero, the tablespace uses the “Antelope” file format (which
is the format used by the standard built-in InnoDB in MySQL up to version 5.1).
Otherwise, the least significant bit should be set in the
tablespace flags, and the file format identifier is written in the
bits 5 through 11. (Divide the tablespace flags by 32 and take the
remainder after dividing the integer part of the result by 128.)
tablename.ibd
Each InnoDB tablespace file (with a name matching
*.ibd) is tagged with the file format used to
create its table and indexes. The way to downgrade the tablespace
is to re-create the table and its indexes. The easiest way to
recreate a table and its indexes is to use the command:
ALTER TABLEtROW_FORMAT=COMPACT;
on each table that you want to downgrade. The COMPACT row format
uses the file format “Antelope”. It was introduced in MySQL
5.0.3.
The file format used by the standard built-in InnoDB in MySQL 5.1 is the “Antelope” format, and the new file format introduced with the InnoDB Plugin 1.0 is the “Barracuda” format. No definitive plans have been made to introduce new features that would require additional new file formats. However, the file format mechanism introduced with the InnoDB Plugin allows for further enhancements.
For the sake of completeness, these are the file format names that might be used for future file formats: Antelope, Barracuda, Cheetah, Dragon, Elk, Fox, Gazelle, Hornet, Impala, Jaguar, Kangaroo, Leopard, Moose, Nautilus, Ocelot, Porpoise, Quail, Rabbit, Shark, Tiger, Urchin, Viper, Whale, Xenops, Yak and Zebra. These file formats correspond to the internal identifiers 0..25.