Mysql：Symbolic Links：软链接（符号链接）

8.12.3 Using Symbolic Links

8.12.3.1 Using Symbolic Links for Databases on Unix

8.12.3.2 Using Symbolic Links for MyISAM Tables on Unix

8.12.3.3 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.

For InnoDB tables, use the DATA DIRECTORY clause of the CREATE TABLE statement instead of symbolic links, as explained in Section 14.6.1.2, “Creating Tables Externally”. This new feature is a supported, cross-platform technique.

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';

8.12.3.1 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, for 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.

8.12.3.2 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:

  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.

8.12.3.3 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.

On Windows, 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.

 

 

 

 

• have_symlink

  YES if symbolic link support is enabled, NO if not. This is required on Unix for support of the DATA DIRECTORY and INDEX DIRECTORY table options. If the server is started with the --skip-symbolic-links option, the value is DISABLED.

  This variable has no meaning on Windows.

• --symbolic-links, --skip-symbolic-links

  Property	Value
  Command-Line Format	--symbolic-links[={OFF|ON}]
  Type	Boolean
  Default Value	ON

  Enable or disable symbolic link support. On Unix, enabling symbolic links means that you can link a MyISAM index file or data file to another directory with the INDEX DIRECTORY or DATA DIRECTORY option of the CREATE TABLE statement. If you delete or rename the table, the files that its symbolic links point to also are deleted or renamed. See Section 8.12.3.2, “Using Symbolic Links for MyISAM Tables on Unix”.

  This option has no meaning on Windows.