Choose the Location of your InnoDB File-per-Table Tablespace

The April 2012 InnoDB labs release introduces a new feature in InnoDB that allows you to choose the location of specific tables.  For example, you can place critical tables onto an SSD drive while leaving the system tablespace on a hard drive.  Conversely, you can store you primary database files on an SSD and put a seldom used but very large archive or reference table on a larger cheaper hard drive.

Innodb now makes use of the following existing syntax in MySQL ;

CREATE TABLE  . . .  DATA DIRECTORY = ‘absolute path of data directory’;

CREATE TABLE  . . .  PARTITION . . . DATA DIRECTORY = ‘absolute path of data directory’;

This syntax is used in MyISAM and Archive engines to make use of symbolic links in those operating systems that support it.  But InnoDB can use this syntax on any OS since it stores the path in a new system table called SYS_DATAFILES.  There is also a new system table called SYS_TABLESPACES.  Both of these can be viewed in the April Labs release of 5.6 by using information schema. For example;

mysql> SET GLOBAL innodb_file_per_table=on;
Query OK, 0 rows affected (0.00 sec)

mysql> CREATE TABLE t1 (a int KEY, b text) DATA DIRECTORY='/ssd1/my_data';
 Query OK, 0 rows affected (0.12 sec)

mysql> SELECT * FROM INFORMATION_SCHEMA.INNODB_SYS_TABLESPACES;
 +-------+---------+------+-------------+----------------------+-----------+---------------+
 | SPACE | NAME    | FLAG | FILE_FORMAT | ROW_FORMAT           | PAGE_SIZE | ZIP_PAGE_SIZE |
 +-------+---------+------+-------------+----------------------+-----------+---------------+
 |     1 | test/t1 |    0 | Antelope    | Compact or Redundant |     16384 |             0 |
 +-------+---------+------+-------------+----------------------+-----------+---------------+
 1 row in set (0.00 sec)

mysql> SELECT * FROM INFORMATION_SCHEMA.INNODB_SYS_DATAFILES;
 +-------+---------------------------+
 | SPACE | PATH                      |
 +-------+---------------------------+
 |     1 | /ssd1/my_data/test/t1.ibd |
 +-------+---------------------------+
 1 row in set (0.00 sec)

Notice that a directory named for the database is added after the DATA DIRECTORY provided.  This is the same as what is done in the normal data directory.  It allows you to chose the same remote location for all your tablespaces, even if the tables in different databases have the same name.

The last 4 columns in innodb_sys_tablespaces are interpretations of the tablespace flags field. Tablespace flags do not distinguish between Compact and Redundant row format, so you see what is displayed above.  If you want to see the row format of the table, innodb_sys_tables now also interprets the table flags;

mysql> SELECT * FROM INFORMATION_SCHEMA.INNODB_SYS_TABLES WHERE name like 'test%';
 +----------+---------+------+--------+-------+-------------+------------+---------------+
 | TABLE_ID | NAME    | FLAG | N_COLS | SPACE | FILE_FORMAT | ROW_FORMAT | ZIP_PAGE_SIZE |
 +----------+---------+------+--------+-------+-------------+------------+---------------+
 |       24 | test/t1 |   65 |      5 |     1 | Antelope    | Compact    |             0 |
 +----------+---------+------+--------+-------+-------------+------------+---------------+
 1 row in set (0.00 sec)

In addition to SYS_DATAFILES, the absolute file path to the tablespace is also written to a text file in the normal datafiles location of the tablespace.  This is also the location of the FRM file written by the MYSQL server.  It has the same name as the FRM except it has the extension ‘.isl’ which stands for InnoDB Symbolic Link.  It is there to support recovery which happens before the system tables are opened.  For the example above, the sole contents of t1.isl are;

/ssd1/my_data/test/t1.ibd

The ISL file also allows a DBA to move one of these remote tablespaces since ALTER TABLE … DATA DIRECTORY =  ‘. . .’ is currently not supported.  In order to move a tablespace to another location, or to port an entire database along with its remote tablespaces, you must edit this ISL file after moving the OS files.  The ISL file is a simple text file that contains the absolute path and filename of the tablespace.  Just change that to the new location and save your changes.  Do not leave a copy of the tablespace in the old location.

If you have a large existing database that uses file-per-table tablespaces (files with ‘.ibd’ extensions), it is possible to move these from the default location next to the FRM file to a remote location such as a new SSD drive.  But you must shut down the MySQL server first. After it is shut down, move the IBD tablespace to the location that you would like, create a text file where it was with the same name except using a ‘isl’ extension.  Make the contents of the text file to be the full file path of the ibd file.  Then restart the server.  It will read the isl file, find the tablespace, and update the internal system tables to show where the file resides.

Conclusion

This new feature provides a valuable option to configure your database to make the best use of the hardware available.