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14.4.5 Creating a File-Per-Table Tablespace Outside the Data Directory

To create a new InnoDB file-per-table tablespace in a specific location outside the MySQL data directory, use the DATA DIRECTORY = absolute_path_to_directory clause of the CREATE TABLE statement.

Plan the location in advance, because you cannot use the DATA DIRECTORY clause with the ALTER TABLE statement. The directory you specify could be on another storage device with particular performance or capacity characteristics, such as a fast SSD or a high-capacity HDD.

Within the destination directory, MySQL creates a subdirectory corresponding to the database name, and within that a .ibd file for the new table. In the database directory beneath the MySQL DATADIR directory, MySQL creates a table_name.isl file containing the path name for the table. The .isl file is treated by MySQL like a symbolic link. (Using actual symbolic links has never been supported for InnoDB tables.)

The following example demonstrates creating a file-per-table tablespace outside the MySQL data directory. It shows the .ibd created in the specified directory, and the .isl created in the database directory beneath the MySQL data directory.

mysql> USE test;
Database changed

mysql> SHOW VARIABLES LIKE 'innodb_file_per_table';
| Variable_name         | Value |
| innodb_file_per_table | ON    |
1 row in set (0.00 sec)

mysql> CREATE TABLE t1 (c1 INT PRIMARY KEY) DATA DIRECTORY = '/alternative/directory';
Query OK, 0 rows affected (0.03 sec)

# MySQL creates a .ibd file for the new table in a subdirectory that corresponding  
# to the database name

db_user@ubuntu:~/alternative/directory/test$ ls

# MySQL creates a .isl file containing the path name for the table in a directory 
# beneath the MySQL data directory

db_user@ubuntu:~/mysql/data/test$ ls
db.opt  t1.frm  t1.isl

As of MySQL 5.7.6, you can also use CREATE TABLE ... TABLESPACE in combination with the DATA DIRECTORY clause to create a file-per-table tablespace outside the MySQL data directory. To do so, you must specify innodb_file_per_table as the tablespace name.

CREATE TABLE t2 (c1 INT PRIMARY KEY) TABLESPACE = innodb_file_per_table 
  DATA DIRECTORY = '/alternative/directory';

You do not have to enable innodb_file_per_table when using this method.

Usage Notes:

  • MySQL initially holds the .ibd file open, preventing you from dismounting the device, but might eventually close the table if the server is busy. Be careful not to accidentally dismount an external device while MySQL is running, or to start MySQL while the device is disconnected. Attempting to access a table when the associated .ibd file is missing causes a serious error that requires a server restart.

    A server restart might fail if the .ibd file is still not at the expected path. In this case, manually remove the table_name.isl file in the database directory, and after restarting perform a DROP TABLE to delete the .frm file and remove the information about the table from the data dictionary.

  • Do not put MySQL tables on an NFS-mounted volume. NFS uses a message-passing protocol to write to files, which could cause data inconsistency if network messages are lost or received out of order.

  • If you use an LVM snapshot, file copy, or other file-based mechanism to back up the .ibd file, always use the FLUSH TABLES ... FOR EXPORT statement first to make sure all changes that were buffered in memory are flushed to disk before the backup occurs.

  • The DATA DIRECTORY clause is a supported alternative to using symbolic links, which has always been problematic and was never supported for individual InnoDB tables.

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