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MySQL 5.6 Reference Manual  /  ...  /  File-Per-Table Tablespaces

14.6.3.2 File-Per-Table Tablespaces

Historically, InnoDB tables were stored in the system tablespace. This monolithic approach was targeted at machines dedicated to database processing, with carefully planned data growth, where any disk storage allocated to MySQL would never be needed for other purposes. The file-per-table tablespace feature provides a more flexible alternative, where each InnoDB table is stored in its own tablespace data file (.ibd file). This feature is controlled by the innodb_file_per_table configuration option, which is enabled by default.

Advantages
  • You can reclaim disk space when truncating or dropping a table stored in a file-per-table tablepace. Truncating or dropping tables stored in the shared system tablespace creates free space internally in the system tablespace data files (ibdata files) which can only be used for new InnoDB data.

    Similarly, a table-copying ALTER TABLE operation on table that resides in a shared tablespace can increase the amount of space used by the tablespace. Such operations may require as much additional space as the data in the table plus indexes. The additional space required for the table-copying ALTER TABLE operation is not released back to the operating system as it is for file-per-table tablespaces.

  • The TRUNCATE TABLE operation is faster when run on tables stored in file-per-table tablepaces.

  • You can store specific tables on separate storage devices, for I/O optimization, space management, or backup purposes. In previous releases, you had to move entire database directories to other drives and create symbolic links in the MySQL data directory, as described in Section 8.12.3, “Using Symbolic Links”. In MySQL 5.6.6 and higher, you can specify the location of each table using the syntax CREATE TABLE ... DATA DIRECTORY = absolute_path_to_directory, as explained in Section 14.6.3.4, “Creating a Tablespace Outside of the Data Directory”.

  • You can run OPTIMIZE TABLE to compact or recreate a file-per-table tablespace. When you run an OPTIMIZE TABLE, InnoDB creates a new .ibd file with a temporary name, using only the space required to store actual data. When the optimization is complete, InnoDB removes the old .ibd file and replaces it with the new one. If the previous .ibd file grew significantly but the actual data only accounted for a portion of its size, running OPTIMIZE TABLE can reclaim the unused space.

  • You can move individual InnoDB tables rather than entire databases.

  • You can copy individual InnoDB tables from one MySQL instance to another (known as the transportable tablespace feature).

  • Tables created in file-per-table tablespaces use the Barracuda file format. The Barracuda file format enables features such as compressed and dynamic row formats. Tables created in the system tablespace cannot use these features. To take advantage of these features for an existing table, enable the innodb_file_per_table setting and run ALTER TABLE t ENGINE=INNODB to place the table in a file-per-table tablespace. Before converting tables, refer to Section 14.6.1.3, “Converting Tables from MyISAM to InnoDB”.

  • You can enable more efficient storage for tables with large BLOB or TEXT columns using the dynamic row format.

  • File-per-table tablespaces may improve chances for a successful recovery and save time when a corruption occurs, when a server cannot be restarted, or when backup and binary logs are unavailable.

  • You can back up or restore individual tables quickly using the MySQL Enterprise Backup product, without interrupting the use of other InnoDB tables. This is beneficial if you have tables that require backup less frequently or on a different backup schedule. See Making a Partial Backup for details.

  • File-per-table tablespaces are convenient for per-table status reporting when copying or backing up tables.

  • You can monitor table size at a file system level, without accessing MySQL.

  • Common Linux file systems do not permit concurrent writes to a single file when innodb_flush_method is set to O_DIRECT. As a result, there are possible performance improvements when using file-per-table tablespaces in conjunction with innodb_flush_method.

  • The system tablespace stores the data dictionary and undo logs, and is limited in size by InnoDB tablespace size limits. See Section 14.6.1.6, “Limits on InnoDB Tables”. With file-per-table tablespaces, each table has its own tablespace, which provides room for growth.

Potential Disadvantages
  • With file-per-table tablespaces, each table may have unused space, which can only be utilized by rows of the same table. This could lead to wasted space if not properly managed.

  • fsync operations must run on each open table rather than on a single file. Because there is a separate fsync operation for each file, write operations on multiple tables cannot be combined into a single I/O operation. This may require InnoDB to perform a higher total number of fsync operations.

  • mysqld must keep one open file handle per table, which may impact performance if you have numerous tables in file-per-table tablespaces.

  • More file descriptors are used.

  • innodb_file_per_table is enabled by default in MySQL 5.6 and higher. You may consider disabling it if backward compatibility with earlier versions of MySQL is a concern. Disabling innodb_file_per_table prevents table-copying ALTER TABLE operations from implicitly moving a table that resides in the system tablespace to a file-per-table tablespace.

    For example, when restructuring the clustered index, the table is re-created using the current setting for innodb_file_per_table. This behavior does not apply to ALTER TABLE operations that use the INPLACE algorithm.

  • If many tables are growing there is potential for more fragmentation which can impede DROP TABLE and table scan performance. However, when fragmentation is managed, having files in their own tablespace can improve performance.

  • The buffer pool is scanned when dropping a file-per-table tablespace, which can take several seconds for buffer pools that are tens of gigabytes in size. The scan is performed with a broad internal lock, which may delay other operations. Tables in the system tablespace are not affected.

  • The innodb_autoextend_increment variable, which defines increment size (in MB) for extending the size of an auto-extending shared tablespace file when it becomes full, does not apply to file-per-table tablespace files, which are auto-extending regardless of the innodb_autoextend_increment setting. The initial extensions are by small amounts, after which extensions occur in increments of 4MB.

Enabling File-Per-Table Tablespaces

The innodb_file_per_table option is enabled by default.

To set the innodb_file_per_table option at startup, start the server with the --innodb-file-per-table command-line option, or add this line to the [mysqld] section of my.cnf:

[mysqld]
innodb_file_per_table=1

You can also set innodb_file_per_table dynamically, while the server is running:

mysql> SET GLOBAL innodb_file_per_table=1;

With innodb_file_per_table enabled, you can store InnoDB tables in a tbl_name.ibd file. Unlike the MyISAM storage engine, with its separate tbl_name.MYD and tbl_name.MYI files for indexes and data, InnoDB stores the data and the indexes together in a single .ibd file. The tbl_name.frm file is still created as usual.

If you disable innodb_file_per_table in your startup options and restart the server, or disable it with the SET GLOBAL command, InnoDB creates new tables inside the system tablespace.

You can always read and write any InnoDB tables, regardless of the file-per-table setting.

To move a table from the system tablespace to its own tablespace, change the innodb_file_per_table setting and rebuild the table:

mysql> SET GLOBAL innodb_file_per_table=1;
mysql> ALTER TABLE table_name ENGINE=InnoDB;
Note

InnoDB always needs the system tablespace because it puts its internal data dictionary and undo logs there. The .ibd files are not sufficient for InnoDB to operate.

When a table is moved out of the system tablespace into its own .ibd file, the data files that make up the system tablespace remain the same size. The space formerly occupied by the table can be reused for new InnoDB data, but is not reclaimed for use by the operating system. When moving large InnoDB tables out of the system tablespace, where disk space is limited, you may prefer to enable innodb_file_per_table and recreate the entire instance using the mysqldump command.


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