MySQL Reference Manual for version 4.0.18.

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5.9 Running Multiple MySQL Servers on the Same Machine

5.9.1 Running Multiple Servers on Windows  
5.9.2 Running Multiple Servers on Unix  
5.9.3 Using Client Programs in a Multiple-Server Environment  

In some cases you might want to run multiple mysqld servers on the same machine. You might want to test a new MySQL release while leaving your existing production setup undisturbed. Or you may want to give different users access to different mysqld servers that they manage themselves. (For example, you might be an Internet service provider that wants to provide independent MySQL installations for different customers.)

To run multiple servers on a single machine, each server must have unique values for several operating parameters. These can be set on the command line or in option files. See section 4.3 Specifying Program Options.

At least the following options must be different for each server:

--port controls the port number for TCP/IP connections. --socket controls the socket file path on Unix and the name of the named pipe on Windows. (It's necessary to specify distinct pipe names on Windows only for those servers that support named pipe connections.) --shared-memory-base-name designates the shared memory name used by a Windows server to allow clients to connect via shared memory. --pid-file indicates the name of the file in which a Unix server writes its process ID.

If you use the following options, they must be different for each server:

If you want more performance, you can also specify the following options differently for each server, to spread load between several physical disks:

Having different temporary directories like above is also recommended because it will be easier for you in case you want to know to which MySQL server a certain temporary file belongs.

Generally, each server should also use a different data directory, which is specified using the --datadir=path option.

Warning: Normally you should never have two servers that update data in the same databases! This may lead to unpleasant surprises if your operating system doesn't support fault-free system locking! If (despite this warning) you run multiple servers using the same data directory and they have logging enabled, you must use the appropriate options to specify log file names that are unique to each server. Otherwise, the servers will try to log to the same files.

This warning against sharing a data directory among servers also applies in an NFS environment. Allowing multiple MySQL servers to access a common data directory over NFS is a bad idea!

Make it easy for yourself: Forget about sharing a data directory among servers over NFS. A better solution is to have one computer that contains several CPUs and use an operating system that handles threads efficiently.

If you have multiple MySQL installations in different locations, normally you can specify the base installation directory for each server with the --basedir=path option to cause each server to use a different data directory, log files, and PID file. (The defaults for all these values are determined relative to the base directory.) In that case, the only other options you need to specify are the --socket and --port options. For example, suppose you install different versions of MySQL using `.tar' file binary distributions. These will install in different locations, so you can start the server for each installation using the command ./bin/mysqld_safe under its corresponding base directory. mysqld_safe will determine the proper --basedir option to pass to mysqld, and you need specify only the --socket and --port options to mysqld_safe.

As discussed in the following sections, it is possible to start additional servers by setting environment variables or by specifying appropriate command-line options. However, if you need to run multiple servers on a more permanent basis, it will be more convenient to use option files to specify for each server those option values that must be unique to it.


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