Symbolic link support as described here, along with the --symbolic-links option that controls it, and is deprecated; expect these to be be removed in a future version of MySQL. In addition, the option is disabled by default.
Symlinks are fully supported only for MyISAM tables. For files used by tables for other storage engines, you may get strange problems if you try to use symbolic links. For InnoDB tables, use the alternative technique explained in Section 15.6.1.2, “Creating Tables Externally” instead.
Do not symlink tables on systems that do not have a fully operational realpath() call. (Linux and Solaris support realpath()). To determine whether your system supports symbolic links, check the value of the have_symlink system variable using this statement:
SHOW VARIABLES LIKE 'have_symlink';
The handling of symbolic links for MyISAM tables works as follows:
In the data directory, you always have the data (.MYD) file and the index (.MYI) file. The data file and index file can be moved elsewhere and replaced in the data directory by symlinks.
You can symlink the data file and the index file independently to different directories.
To instruct a running MySQL server to perform the symlinking, use the DATA DIRECTORY and INDEX DIRECTORY options to CREATE TABLE. See Section 13.1.20, “CREATE TABLE Statement”. Alternatively, if mysqld is not running, symlinking can be accomplished manually using ln -s from the command line.
The path used with either or both of the DATA DIRECTORY and INDEX DIRECTORY options may not include the MySQL data directory. (Bug #32167)
myisamchk does not replace a symlink with the data file or index file. It works directly on the file to which the symlink points. Any temporary files are created in the directory where the data file or index file is located. The same is true for the ALTER TABLE, OPTIMIZE TABLE, and REPAIR TABLE statements.
When you drop a table that is using symlinks, both the symlink and the file to which the symlink points are dropped. This is an extremely good reason not to run mysqld as the root operating system user or permit operating system users to have write access to MySQL database directories.
If you rename a table with ALTER TABLE ... RENAME or RENAME TABLE and you do not move the table to another database, the symlinks in the database directory are renamed to the new names and the data file and index file are renamed accordingly.
If you use ALTER TABLE ... RENAME or RENAME TABLE to move a table to another database, the table is moved to the other database directory. If the table name changed, the symlinks in the new database directory are renamed to the new names and the data file and index file are renamed accordingly.
If you are not using symlinks, start mysqld with the --skip-symbolic-links option to ensure that no one can use mysqld to drop or rename a file outside of the data directory.
These table symlink operations are not supported:
ALTER TABLE ignores the DATA DIRECTORY and INDEX DIRECTORY table options.