SpirirHost Support Centre: MySQL Reference Manual p.374

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Table of Contents
1. General Information
2. MySQL Installation
3. Tutorial Introduction
4. Database Administration
5. MySQL Optimisation
6. MySQL Language Reference
7. MySQL Table Types
8. MySQL APIs
9. Extending MySQL

374 MySQL Technical Reference for Version 4.0.3 5.6.1.1 Using Symbolic Links for Databases The way to symlink a database is to rst create a directory on some disk where you have free space and then create a symlink to it from the MySQL database directory. shell> mkdir /dr1/databases/test shell> ln -s /dr1/databases/test mysqld-datadir MySQL doesn't support that you link one directory to multiple databases. Replacing a database directory with a symbolic link will work ne as long as you don't make a symbolic link between databases. Suppose 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 Now, for any table tbl_a in db1, there also appears to be a table tbl_a in db2. If one thread updates db1.tbl_a and another thread updates db2.tbl_a, there will be problems. If you really need this, you must change the following code in `mysys/mf_format.c': if (flag & 32 || (!lstat(to,&stat_buff) && S_ISLNK(stat_buff.st_mode))) to if (1) On Windows you can use internal symbolic links to directories by compiling MySQL with -DUSE_SYMDIR. This allows you to put dierent databases on dierent disks. See Sec- tion 2.6.2.5 [Windows symbolic links], page 117. 5.6.1.2 Using Symbolic Links for Tables Before MySQL 4.0 you should not symlink tables, if you are not very careful with them. The problem is that if you run ALTER TABLE, REPAIR TABLE or OPTIMIZE TABLE on a symlinked table, the symlinks will be removed and replaced by the original les. This happens because the above command works by creating a temporary le in the database directory and when the command is complete, replace the original le with the temporary le. You should not symlink tables on systems that don't have a fully working realpath() call. (At least Linux and Solaris support realpath()) In MySQL 4.0 symlinks are only fully supported for MyISAM tables. For other table types you will probably get strange problems when doing any of the above mentioned commands. The handling of symbolic links in MySQL 4.0 works the following way (this is mostly relevant only for MyISAM tables). In the data directory you will always have the table denition le and the data/index les. You can symlink the index le and the datale to dierent directories independent of the other. The symlinking can be done from the operating system (if mysqld is not running) or with the INDEX/DATA DIRECTORY="path-to-dir" command in CREATE TABLE. See Section 6.5.3 [CREATE TABLE], page 469.

MySQL Reference Manual

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