Using Symbolic Links for Tables on Unix

Do not symlink tables on systems that do not have a fully operational realpath() call. (Linux and Solaris support realpath()). Check whether your system supports symbolic links by issuing a SHOW VARIABLES LIKE 'have_symlink' statement.

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.

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.
• You can instruct a running MariaDB server to perform the symlinking by using the DATA DIRECTORY and INDEX DIRECTORY options to CREATE TABLE. See , "CREATE TABLE Syntax". Alternatively, symlinking can be accomplished manually from the command line using ln -s if mysqld is not running.Note

  The path used with either or both of the DATA DIRECTORY and INDEX DIRECTORY options may not include the MariaDB 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 MariaDB 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, use the --skip-symbolic-links option to mysqld to ensure that no one can use mysqld to drop or rename a file outside of the data directory.

Table symlink operations that are not yet supported:

• ALTER TABLE ignores the DATA DIRECTORY and INDEX DIRECTORY table options.
• The .frm file must never be a symbolic link (as indicated previously, only the data and index files can be symbolic links). Attempting to do this (for example, to make synonyms) produces incorrect results. Suppose that you have a database db1 under the MariaDB 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.

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