MySQL DBA Training Session 20. Optimizing MySQL ServerRAM N SANGWAN

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Agenda• Introduction to Status Variables

• Interpreting mysqld Server Information

• Accessing Server System Variables

• Setting System Variables

• Static Vs Dynamic Variables

• Global Variables

• Session Variables

• Display Variables

• Accessing Server Status Variables

• Understanding the Values of StatusVariables

• Measuring Server Load

• Process List

• Tuning Memory Parameters

• Overall mysqld options

• Configure table caching

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Introduction

• You have to have a understanding of the status command and the globalvariables that are associated.

• The status command produces over 250 variables.

Interpreting mysqld Server Information

SHOW VARIABLES

◦ displays server system variables.

◦ These indicate such things as directory locations, server capabilities, andsizes of caches and buffers.

◦ You can set system variables to control how the server operates.

◦ They can be set at server startup, and many of them can be changed whilethe server is running.

Interpreting mysqld Server Information

SHOW STATUS

◦ displays server status variables that indicate the extent and types of activitiesthe server is performing.

◦ These variables provide information such as how long the server has beenrunning, number of queries processed, amount of network traffic, andstatistics about the query cache.

◦ You can use status information to assess how much of a load your server isprocessing and how well it is handling the load.

◦ This information provides useful feedback for assessing whether systemvariables should be changed to improve server performance.

Accessing Server System Variables

• To display these variables, use the SHOW VARIABLES statement:

mysql> SHOW VARIABLES;

• To display only those variables with names that match a given pattern, add aLIKE pattern-matching clause.

• The pattern is not case sensitive and may contain the '%' and '_' wildcardpattern metacharacters:

mysql> SHOW VARIABLES LIKE '%buffer_size';

• If the pattern contains no metacharacters, the statement displays only thenamed variable:

mysql> SHOW VARIABLES LIKE 'datadir';

Accessing Server System Variables

• System variables may be displayed in other ways as well.

• mysqladmin variables provides command-line access to the complete list ofsystem variables.

• Both clients implement this capability by sending a SHOW VARIABLESstatement to the server and displaying the results.

Setting System Variables

• System variables can be set at server startup using options on the commandline or in option files.

• For example, on a Unix machine, you can put the following lines in the/etc/my.cnf option file to specify a data directory of /var/mysql/data and a keycache size of 64MB:

[mysqld]

datadir = /var/mysql/data

key_buffer_size = 64M

Static Vs Dynamic Variables

• Some server system variables are static and can only be set at startup time.

• For example, you can specify the data directory by means of a datadir startup option, but youcannot tell a server that is running to use a different data directory.

• Other variables are dynamic and can be changed while the server is running:

mysql> SET GLOBAL key_buffer_size = 128*1024*1024;

mysql> SET @@global.key_buffer_size = 128*1024*1024;

• With a SET statement, you cannot use a suffix of K, M, or G to indicate units for the value, butyou can use an expression

• To set global variables, you must have the SUPER privilege.

Global Variables

• An example of the type of variable that has both forms is storage_engine,which controls the default storage engine used for CREATE TABLEstatements that do not specify a storage engine explicitly.

• The global storage_engine value is used to set the session storage_enginevariable for each client when the client connects, but the client may change itssession variable value to use a different default storage engine.

Session Variables

• For example, the default storage engine may be set either globally or only for the currentconnection using the following statements:

mysql> SET GLOBAL storage_engine = MyISAM;

mysql> SET @@global.storage_engine = MyISAM;

mysql> SET SESSION storage_engine = InnoDB;

mysql> SET @@session.storage_engine = InnoDB;

• LOCAL is a synonym for SESSION.

• Also, if you do not indicate explicitly whether to set the global or session version of a variable,MySQL sets the session variable.

Display Variables

• To explicitly display global or session variable values, use SHOW GLOBAL VARIABLES or SHOW SESSION VARIABLES.

• Without GLOBAL or SESSION, the SHOW VARIABLES statement displays session values.

• It's also possible to use SELECT to display the values of individual global or session values:

mysql> SELECT @@global.storage_engine, @@session.storage_engine;

Accessing Server Status Variables

• The server makes the current values of these variables available through theSHOW STATUS:

mysql> SHOW STATUS;

• To display only those variables with names that match a given pattern, add aLIKE pattern-matching clause. :

mysql> SHOW STATUS LIKE 'qcache%';

• Status variables may be obtained in other ways as well. mysqladminextended-status provides command-line access to the complete list of statusvariables, and mysqladmin status displays a brief summary.

Understanding the Values of Status Variables

• Several status variables provide information about how many connections theserver is handling, including the number of successful and unsuccessfulconnection attempts, and also whether successful connections terminatenormally or abnormally.◦ The total number of connection attempts: Connections

◦ The number of unsuccessful connection attempts: Aborted_connects

◦ The number of successful connection attempts: Connections - Aborted_connects

◦ The number of successful connections that terminated abnormally: Aborted_clients

◦ The successful connections terminated normally: Connections - Aborted_connects -Aborted_clients

◦ The number of clients currently connected to the server: Threads_connected

Measuring Server Load

• Several status variables displayed by SHOW STATUS provide loadinformation. For example,

• Questions indicates the number of queries the server has processed and

• Uptime indicates the number of seconds the server has been running.

• Combining these, the ratio Questions/ Uptime tells you how many queries persecond the server has processed.

Measuring Server Load

• slow_queries indicates the number of queries that take a long time toprocess.

• Ideally, its value should increase slowly or not at all.

• If it increases quickly, you might have a problem with certain queries.

• The slow query log shows the text of slow queries and provides informationabout how long they took.

• Restart the server with the slow query log enabled, let it run for a while, andthen take a look at the log to see which queries turn up there.

• You can use this log to identify queries that might need optimizing.

Process List

• SHOW PROCESSLIST displays information about the activity of eachcurrently connected client.

• The SHOW PROCESSLIST statement always shows your own queries.

• If you have the PROCESS privilege, it also shows queries being run by otheraccounts.

• To get a concise report of the server's load status from within the mysql clientprogram, use its STATUS (or \s) command to display a general snapshot ofthe current connection state.

• The last part of the output provides some information about the server load:

mysql> STATUS;

• The final part of the output also can be obtained by issuing a mysqladminstatus command.

Tuning Memory Parameters

• A small system may use options with small values:

[mysqld]

key_buffer_size = 16K

table_cache = 4

sort_buffer_size = 64K

• For a larger system, you can increase the values, and also allocate memory to the query cache:

[mysqld]

key_buffer_size = 256M

table_cache = 256

sort_buffer_size = 1M

query_cache_type = ON

query_cache_size = 16M

Overall mysqld options

memlock

• Locks the mysqld daemon into server memory, it can improve performancebut if the server runs out of memory then mysqld daemon will crash.

sort_buffer_size

• Determines the amount of memory allocated for SQL sorts, if this space isexceeded then the server will use hard disk space resulting in worseperformance.

Overall mysqld options Contd..

thread_cache_size

• mysqld creates a cache of unused connection thread rather than destroyingthreads and creating new ones, increase this variable if you have a lot ofusers logging in and out of the MySQL server.

thread_concurrency

• only use on Solaris systems, this should be set to twice the number of CPU's.

tmp_table_size

• both this and max_heap_table_size are used to determine the maximum sizeallowed of an in-memory temporary table before it is converted to a MyISAMtable, the smallest value of either one is used.

Configure table caching

• MySQL has at least one file per table : the FRM file

table_open_cache

◦ not too small, not too big

◦ opened_tables / sec

table_definition_cache

◦ do not forget to increase

◦ opened_table_definitions / sec

table_cache_instances = 8 or 16

innodb_open_files

mdl_hash_instances = 256

Thank You