8.12.3.2 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. For InnoDB tables, use the alternative technique explained in Section 14.6.1.2, “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 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 13.1.18, “CREATE TABLE Statement”. 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 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.

• 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:

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