8.11.3.1 Using Symbolic Links

8.11.3.1.1 Using Symbolic Links for Databases on Unix
8.11.3.1.2 Using Symbolic Links for MyISAM Tables on Unix
8.11.3.1.3 Using Symbolic Links for Databases on Windows

You can move databases or tables from the database directory to other locations and replace them with symbolic links to the new locations. You might want to do this, for example, to move a database to a file system with more free space or increase the speed of your system by spreading your tables to different disks.

The recommended way to do this is to symlink entire database directories to a different disk. Symlink MyISAM tables only as a last resort.

To determine the location of your data directory, use this statement:

SHOW VARIABLES LIKE 'datadir';
8.11.3.1.1 Using Symbolic Links for Databases on Unix

On Unix, the way to symlink a database is first to create a directory on some disk where you have free space and then to create a soft link to it from the MySQL data directory.

shell> mkdir /dr1/databases/test
shell> ln -s /dr1/databases/test /path/to/datadir

MySQL does not support linking one directory to multiple databases. Replacing a database directory with a symbolic link works as long as you do not make a symbolic link between databases. Suppose that you have a database db1 under the MySQL data directory, and then make a symlink db2 that points to db1:

shell> cd /path/to/datadir
shell> ln -s db1 db2

The result is that, or any table tbl_a in db1, there also appears to be a table tbl_a in db2. If one client updates db1.tbl_a and another client updates db2.tbl_a, problems are likely to occur.

8.11.3.1.2 Using Symbolic Links for MyISAM Tables on Unix

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.

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 table format (.frm) file, 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. The format file cannot.

  • 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 13.1.17, “CREATE TABLE Syntax”. Alternatively, if mysqld is not running, symlinking can be accomplished manually using ln -s from the command line.

    Note

    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.

  • Note

    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 system root or permit 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.

  • As indicated previously, only the data and index files can be symbolic links. The .frm file must never be a symbolic link. Attempting to do this (for example, to make one table name a synonym for another) produces incorrect results. Suppose that you have a database db1 under the MySQL data directory, a table tbl1 in this database, and in the db1 directory you make a symlink tbl2 that points to tbl1:

    shell> cd /path/to/datadir/db1
    shell> ln -s tbl1.frm tbl2.frm
    shell> ln -s tbl1.MYD tbl2.MYD
    shell> ln -s tbl1.MYI tbl2.MYI
    

    Problems result if one thread reads db1.tbl1 and another thread updates db1.tbl2:

    • The query cache is fooled (it has no way of knowing that tbl1 has not been updated, so it returns outdated results).

    • ALTER statements on tbl2 fail.

8.11.3.1.3 Using Symbolic Links for Databases on Windows

On Windows, symbolic links can be used for database directories. This enables you to put a database directory at a different location (for example, on a different disk) by setting up a symbolic link to it. Use of database symlinks on Windows is similar to their use on Unix, although the procedure for setting up the link differs.

Suppose that you want to place the database directory for a database named mydb at D:\data\mydb. To do this, create a symbolic link in the MySQL data directory that points to D:\data\mydb. However, before creating the symbolic link, make sure that the D:\data\mydb directory exists by creating it if necessary. If you already have a database directory named mydb in the data directory, move it to D:\data. Otherwise, the symbolic link will be ineffective. To avoid problems, make sure that the server is not running when you move the database directory.

The procedure for creating the database symbolic link depends on your version of Windows.

Windows Vista, Windows Server 2008, or newer have native symbolic link support, so you can create a symlink using the mklink command. This command requires administrative privileges.

  1. Change location into the data directory:

    C:\> cd \path\to\datadir
    
  2. In the data directory, create a symlink named mydb that points to the location of the database directory:

    C:\> mklink /d mydb D:\data\mydb
    

After this, all tables created in the database mydb are created in D:\data\mydb.

Alternatively, on any version of Windows supported by MySQL, you can create a symbolic link to a MySQL database by creating a .sym file in the data directory that contains the path to the destination directory. The file should be named db_name.sym, where db_name is the database name.

Support for database symbolic links on Windows using .sym files is enabled by default. If you do not need .sym file symbolic links, you can disable support for them by starting mysqld with the --skip-symbolic-links option. To determine whether your system supports .sym file symbolic links, check the value of the have_symlink system variable using this statement:

SHOW VARIABLES LIKE 'have_symlink';

To create a .sym file symlink, use this procedure:

  1. Change location into the data directory:

    C:\> cd \path\to\datadir
    
  2. In the data directory, create a text file named mydb.sym that contains this path name: D:\data\mydb\

    Note

    The path name to the new database and tables should be absolute. If you specify a relative path, the location will be relative to the mydb.sym file.

After this, all tables created in the database mydb are created in D:\data\mydb.

The following limitations apply to the use of .sym files for database symbolic linking on Windows. These limitations do not apply for symlinks created using mklink.

  • The symbolic link is not used if a directory with the same name as the database exists in the MySQL data directory.

  • The --innodb_file_per_table option cannot be used.

  • If you run mysqld as a service, you cannot use a mapped drive to a remote server as the destination of the symbolic link. As a workaround, you can use the full path (\\servername\path\).