There are different reasons for creating
tables externally; that is, creating tables outside of the data
directory. Those reasons might include space management, I/O
optimization, or placing tables on a storage device with
particular performance or capacity characteristics, for example.
You can create an
InnoDB table in an external
directory by specifying a
DATA DIRECTORY clause
CREATE TABLE statement.
CREATE TABLE t1 (c1 INT PRIMARY KEY) DATA DIRECTORY = '/external/directory';
DATA DIRECTORY clause is supported for
tables created in file-per-table tablespaces. Tables are
implicitly created in file-per-table tablespaces when the
innodb_file_per_table variable is
enabled, which it is by default.
mysql> SELECT @@innodb_file_per_table; +-------------------------+ | @@innodb_file_per_table | +-------------------------+ | 1 | +-------------------------+
For more information about file-per-table tablespaces, see Section 126.96.36.199, “File-Per-Table Tablespaces”.
Be sure of the directory location you choose, as the
DIRECTORY clause cannot be used with
ALTER TABLE to change the location
When you specify a
DATA DIRECTORY clause in a
CREATE TABLE statement, the table's data file
is created in a schema directory under the specified directory,
that contains the data file path is created in the schema
directory under the MySQL data directory. An
.isl file is similar in function to a
symbolic link. (Actual symbolic
links are not supported for use with
InnoDB data files.)
The following example demonstrates creating a table in an external
directory using the
DATA DIRECTORY clause. It
is assumed that the
innodb_file_per_table variable is
mysql> USE test; Database changed mysql> CREATE TABLE t1 (c1 INT PRIMARY KEY) DATA DIRECTORY = '/external/directory'; # MySQL creates the table's data file in a schema directory # under the external directory $> cd /external/directory/test $> ls t1.ibd # An .isl file that contains the data file path is created # in the schema directory under the MySQL data directory $> cd /path/to/mysql/data/test $> ls db.opt t1.frm t1.isl
MySQL initially holds the tablespace data file open, preventing you from dismounting the device, but might eventually close the file if the server is busy. Be careful not to accidentally dismount an external device while MySQL is running, or start MySQL while the device is disconnected. Attempting to access a table when the associated data file is missing causes a serious error that requires a server restart.
A server restart might fail if the data file is not found at the expected path. In this case, manually remove the
.islfile from the schema directory. After restarting, drop the table to remove the
.frmfile and the information about the table from the data dictionary.
Before placing a table on an NFS-mounted volume, review potential issues outlined in Using NFS with MySQL.
If using an LVM snapshot, file copy, or other file-based mechanism to back up the table's data file, always use the
FLUSH TABLES ... FOR EXPORTstatement first to ensure that all changes buffered in memory are flushed to disk before the backup occurs.
DATA DIRECTORYclause to create a table in an external directory is an alternative to using symbolic links, which
InnoDBdoes not support.
DATA DIRECTORYclause is not supported in a replication environment where the source and replica reside on the same host. The
DATA DIRECTORYclause requires a full directory path. Replicating the path in this case would cause the source and replica to create the table in same location.