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19 Solving some common problems with MySQL

19.1 Database replication

One way replication can be used both to increase robustness and speed. For robustness you have two systems and switch to the backup if you get problems you witch to the backup. The extra speed is achieved by sending a part of the non updating queries to the replica server. Of course this only works if non updating queries dominate, but that is the normal case.

One way replication is planned for the near future. This will be implemented so that slave servers will be synchronized with low priority updates and delayed inserts up to date (this will give readers higher priority than writers).

MySQL doesn't (yet) have database replication, but here are some info on how do to it.

The most general way to replicate a database is to use the update log. See section 9.2 The update log. This requires one database that acts as a master (to which data changes are made) and one or more other databases that act as slaves. To update a slave, just run mysql < update_log. Supply host, user and password options that are appropriate for the slave database, and use the update log from the master database as input.

If you never delete anything from a table, you can use a TIMESTAMP column to find out which rows have been inserted or changed in the table since the last replication (by comparing to the time when you did the replication last time) and only copy these rows to the mirror.

It is possible to make a two-way updating system using both the update log (for deletes) and timestamps (on both sides). But in that case you must be able to handle conflicts when the same data have been changed in both ends. You probably want to keep the old version to help with deciding what has been updated.

Because replication in this case is done with SQL statements, you should not use the following functions in statements that update the database; they may not return the same value as in the original database:

  • DATABASE()
  • GET_LOCK() and RELEASE_LOCK()
  • RAND()
  • USER(), SYSTEM_USER() or SESSION_USER()
  • VERSION()

All time functions are safe to use, as the timestamp is sent to the mirror if needed. LAST_INSERT_ID() is also safe to use.

19.2 Database backups

Since MySQL tables are stored as files, it is easy to do a backup. To get a consistent backup, do a LOCK TABLES on the relevant tables. See section 7.24 LOCK TABLES/UNLOCK TABLES syntax. You only need a read lock; this allows other threads to continue to query the tables while you are making a copy of the files in the database directory. If you want to make a SQL level backup, you can use SELECT INTO OUTFILE.

Another way to backup a database is to use the mysqldump program:

  1. Do a full backup of your databases:
    shell> mysqldump --tab=/path/to/some/dir --opt --full
    
    You can also simply copy all table files (`*.frm', `*.MYD' and `*.MYI' files), as long as the server isn't updating anything.
  2. Stop mysqld if it's running, then start it with the --log-update option. You will get log files with names of the form `hostname.n', where n is a number that is incremented each time you execute mysqladmin refresh or mysqladmin flush-logs, the FLUSH LOGS statement, or restart the server. These log files provide you with the information you need to replicate changes to the database that are made subsequent to the point at which you executed mysqldump.

If you have to restore something, try to recover your tables using myisamchk -r first. That should work in 99.9% of all cases. If myisamchk fails, try the following procedure:

  1. Restore the original mysqldump backup.
  2. Execute the following command to re-run the updates in the update logs:
    shell> ls -1 -t -r hostname.[0-9]* | xargs cat | mysql
    

ls is used to get all the log files in the right order.

You can also do selective backups with SELECT * INTO OUTFILE 'file_name' FROM tbl_name and restore with LOAD DATA INFILE 'file_name' REPLACE ... To avoid duplicate records, you need a PRIMARY KEY or a UNIQUE key in the table. The REPLACE keyword causes old records to be replaced with new ones when a new record duplicates an old record on a unique key value.

19.3 Running multiple MySQL servers on the same machine

There are circumstances when you might want to run multiple servers on the same machine. For example, you might want to test a new MySQL release while leaving your existing production setup undisturbed. Or you might be an Internet service provider that wants to provide independent MySQL installations for different customers.

If you want to run multiple servers, the easiest way is to compile the servers with different TCP/IP ports and socket files so they are not both listening to the same TCP/IP port or socket file.

Assume an existing server is configured for the default port number and socket file. Then configure the new server with a configure command something like this:

shell> ./configure  --with-tcp-port=port_number 
             --with-unix-socket=file_name 
             --prefix=/usr/local/mysql-3.22.9

Here port_number and file_name should be different than the default port number and socket file pathname, and the --prefix value should specify an installation directory different than the one under which the existing MySQL installation is located.

You can check the socket and port used by any currently-executing MySQL server with this command:

shell> mysqladmin -h hostname --port=port_number variables

If you have a MySQL server running on the port you used, you will get a list of some of the most important configurable variables in MySQL, including the socket name.

You should also edit the initialization script for your machine (probably `mysql.server') to start and kill multiple mysqld servers.

You don't have to recompile a new MySQL server just to start with a different port and socket. You can change the port and socket to be used by specifying them at runtime as options to safe_mysqld:

shell> /path/to/safe_mysqld --socket=file_name --port=port_number

If you run the new server on the same database directory as another server with logging enabled, you should also specify the name of the log files to safe_mysqld with --log and --log-update. Otherwise, both servers may be trying to write to the same log file.

Warning: Normally you should never have two servers that update data in the same database! If your OS doesn't support fault-free system locking, this may lead to unpleasant surprises!

If you want to use another database directory for the second server, you can use the --datadir=path option to safe_mysqld.

When you want to connect to a MySQL server that is running with a different port than the port that is compiled into your client, you can use one of the following methods:

  • Start the client with --host 'hostname' --port=port_numer or [--host localhost] --socket=file_name.
  • In your C or Perl programs, you can give the port and socket arguments when connecting to the MySQL server.
  • Set the MYSQL_UNIX_PORT and MYSQL_TCP_PORT environment variables to point to the Unix socket and TCP/IP port before you start your clients. If you normally use a specific socket or port, you should place commands to set these environment variables in your `.login' file. See section 12.1 Overview of the different MySQL programs.
  • Specify the default socket and TCP/IP port in the `.my.cnf' file in your home directory. See section 4.15.4 Option files.


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