MySQL Reference Manual for version 4.0.18.

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7.5.2 Tuning Server Parameters

You can determine the default buffer sizes used by the mysqld server with this command:

 
shell> mysqld --help

This command produces a list of all mysqld options and configurable variables. The output includes the default variable values and looks something like this:

 
back_log                 current value: 5
bdb_cache_size           current value: 1048540
binlog_cache_size        current value: 32768
connect_timeout          current value: 5
delayed_insert_timeout   current value: 300
delayed_insert_limit     current value: 100
delayed_queue_size       current value: 1000
flush_time               current value: 0
interactive_timeout      current value: 28800
join_buffer_size         current value: 131072
key_buffer_size          current value: 1048540
lower_case_table_names   current value: 0
long_query_time          current value: 10
max_allowed_packet       current value: 1048576
max_binlog_cache_size    current value: 4294967295
max_connections          current value: 100
max_connect_errors       current value: 10
max_delayed_threads      current value: 20
max_heap_table_size      current value: 16777216
max_join_size            current value: 4294967295
max_sort_length          current value: 1024
max_tmp_tables           current value: 32
max_write_lock_count     current value: 4294967295
myisam_sort_buffer_size  current value: 8388608
net_buffer_length        current value: 16384
net_retry_count          current value: 10
net_read_timeout         current value: 30
net_write_timeout        current value: 60
read_buffer_size         current value: 131072
read_rnd_buffer_size     current value: 262144
slow_launch_time         current value: 2
sort_buffer              current value: 2097116
table_cache              current value: 64
thread_concurrency       current value: 10
tmp_table_size           current value: 1048576
thread_stack             current value: 131072
wait_timeout             current value: 28800

If there is a mysqld server currently running, you can see what values it actually is using for the variables by issuing this statement:

 
mysql> SHOW VARIABLES;

You can also see some statistics and status indicators for a running server by issuing this statement:

 
mysql> SHOW STATUS;

You can find a full description for all variables in the SHOW VARIABLES section in this manual. See section SHOW VARIABLES. For information about status variables, see SHOW STATUS.

Server variable and status information also can be obtained using mysqladmin:

 
shell> mysqladmin variables
shell> mysqladmin extended-status

MySQL uses algorithms that are very scalable, so you can usually run with very little memory. However, if you give MySQL more memory, normally you will also get better performance.

When tuning a MySQL server, the two most important variables to use are key_buffer_size and table_cache. You should first feel confident that you have these set appropriately before trying to change any other variables.

The following examples indicate some typical variable values for different runtime configurations. The examples use the mysqld_safe script and use --name=value syntax to set the variable name to the value value. This syntax is available as of MySQL 4.0. For older versions of MySQL, take the following differences into account:

If you have at least 256M of memory and many tables and want maximum performance with a moderate number of clients, you should use something like this:

 
shell> mysqld_safe --key_buffer_size=64M --table_cache=256 \
           --sort_buffer_size=4M --read_buffer_size=1M &

If you have only 128M of memory and only a few tables, but you still do a lot of sorting, you can use something like:

 
shell> mysqld_safe --key_buffer_size=16M --sort_buffer_size=1M

If you have little memory and lots of connections, use something like this:

 
shell> mysqld_safe --key_buffer_size=512K --sort_buffer_size=100K \
           --read_buffer_size=100K &

Or even this:

 
shell> mysqld_safe --key_buffer_size=512K --sort_buffer_size=16K \
           --table_cache=32 --read_buffer_size=8K -O net_buffer_length=1K &

If you are doing a GROUP BY or ORDER BY on tables that are much larger than your available memory, you should increase the value of read_rnd_buffer_size to speed up the reading of rows after sorting operations.

When you have installed MySQL, the `support-files' directory will contain some different `my.cnf' example files, `my-huge.cnf', `my-large.cnf', `my-medium.cnf', and `my-small.cnf', you can use as a base to optimize your system.

If there are very many simultaneous connections, swapping problems may occur unless mysqld has been configured to use very little memory for each connection. mysqld performs better if you have enough memory for all connections.

Note that if you specicy an option on the command line for mysqld or mysqld_safe, it remains in effect only for that invocation of the server. To use the option every time the server runs, put it in an option file.

To see the effects of a parameter change, do something like this:

 
shell> mysqld --key_buffer_size=32m --help

Make sure that the --help option is last; otherwise, the effect of any options listed after it on the command line will not be reflected in the output.


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