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382 Part III . Administration Some of the same counters are included in the statusand extended-status forms. (Chapter 10 has more information on the specific values.) MySQL CLI troubleshooting and performance functions The MySQL CLI includes many of the same functions as the mysqladmin command . For example, the mysqladmin flush-logs function is replaced with the FLUSH LOGS statement when executed with the CLI. The mysqladmin status command is replaced with the SHOW STATUS statement when used with the CLI. Table 14-4 shows mysqladmin functions with their CLI equivalents. Table 14-4 Functions with mysqladmin and the MySQL CLI Mysqladmin function MySQL CLI equivalent Variables SHOW VARIABLES Status SHOW STATUS Processlist SHOW PROCESSLIST Flush-hosts FLUSH HOSTS Flush-privileges FLUSH PRIVILEGES Flush-logs FLUSH LOGS Flush-tables FLUSH TABLES Flush-status FLUSH STATUS When run from within the CLI, the FLUSH TABLESstatement has three variations. The first is the normal FLUSH TABLES statement that closes all open tables. The second variation closes only specified tables and can be called in singular or plural form: FLUSH [TABLE | TABLES]
(,
) Two examples of the FLUSH TABLESstatement with specific tables are shown in Figure 14-8.
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Chapter 14 . Performance Tuning 381 | Aborted_connects | Bytes_received | Bytes_sent | Connections | Created_tmp_disk_tables | Created_tmp_tables | Created_tmp_files | Delayed_insert_threads | Delayed_writes | Delayed_errors | Flush_commands | Handler_delete | Handler_read_first | Handler_read_key | Handler_read_next | Handler_read_prev | Handler_read_rnd | Handler_read_rnd_next | Handler_update | Handler_write | Key_blocks_used | Key_read_requests | Key_reads | Key_write_requests | Key_writes | Max_used_connections | Not_flushed_key_blocks | 0 | | 280 | | 5745 | | 7 | | 0 | | 0 | | 0 | | 0 | | 0 | | 0 | | 1 | | 0 | | 1 | | 2 | | 8 | | 0 | | 0 | | 60 | | 0 | | 0 | | 3 | | 8 | | 3 | | 0 | | 0 | | 0 | | 0 | | Not_flushed_delayed_rows | 0 | | Open_tables | Open_files | Open_streams | Opened_tables | Questions | Select_full_join | Select_full_range_join | Select_range | Select_range_check | Select_scan | Slave_running | Slave_open_temp_tables | Slow_launch_threads | Slow_queries | Sort_merge_passes | Sort_range | Sort_rows | Sort_scan | Table_locks_immediate | Table_locks_waited | Threads_cached | Threads_created | Threads_connected | Threads_running | 1 | | 6 | | 0 | | 7 | | 10 | | 0 | | 0 | | 0 | | 0 | | 1 | | OFF | | 0 | | 0 | | 0 | | 0 | | 0 | | 0 | | 0 | | 6 | | 0 | | 0 | | 6 | | 1 | | 1 | | Uptime | 145212 | +————————–+——–+
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380 Part III . Administration Table 14-3 Status and Variable functions with mysqladmin Function Purpose extended-status Prints the current value for many server counters and status-related objects. ping Checks to see if the mysqld process is still alive. processlist Lists the currently open processes, along with their status and action. status Prints a short status message on the server with some status-related objects. variables Outputs the current values for the variables set for the MySQL server. The status and extended-statusfunctions are related because they both output the current status of various counters within the server. The status message shows relatively few important objects; the output of extended-status shows many objects. Tip Unless you have an insanely small resolution on a huge monitor, you ll probably need to pipe or redirect the output from the extended-status function so it doesn t scroll by too fast on-screen. Figure 14-7 shows an example of the status function. The full output from the extended-status function follows as well. Figure 14-7: Output from the mysqladmin status function shows the current values for important counters. Compare the output from the status function in Figure 14-7 to the following output from the extended-status function. +————————–+——–+ | Variable_name | Value | +————————–+——–+ | Aborted_clients | 0 |
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Chapter 14 . Performance Tuning 379 Other flush functions with mysqladmin mysqladmin has five other functions for flushing objects. These are shown in Table 14-2. Table 14-2 Flush functions with mysqladmin Function name Purpose flush-hosts Clears the DNS cache of MySQL. Can assist if a host gets blocked due to errors. flush-logs Clears and cycles MySQL logfiles. flush-privileges Reloads the MySQL grants database so privilege changes take effect. flush-status Clears server counters found with the status and extended- status functions. flush-tables Closes all open tables. flush-threads Closes all open threads. Cross-The flush functions are discussed in detail in Chapter 10. Reference Two flush functions have synonyms or aliases. . The reload function is an alias for the flush-privilegesfunction. Both reload the grant tables. . The refresh function is an alias that performs both the duties of the flush- tables function and the flush-logs function. Running a mysqladmin refresh will close all open tables and restart logging (or cycle the logfiles). Status and variable functions In addition to the functions that flush or reset values and objects with mysqladmin, other functions examine server status and variables. Cross-The flush functions are discussed in detail in Chapter 10. Reference The functions and their purposes are shown in Table 14-3.
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378 Part III . Administration Functions to clear logfiles Logfiles can be reset or rotated to the next logfile through the flush-logsfunction. For example, Figure 14-5 shows a directory listing of the numerous logfiles running on this server. Figure 14-6 shows the mysqladmin flush-logs function being run, followed by another directory listing. In that case, the binary log has been rotated to number 004 in the sequence and the server has also accessed other logs. Figure 14-5: A directory listing showing the logfiles and timestamps before the flush-logs function is run Figure 14-6: The mysqladmin flush-logs function is run along with another directory listing.
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Chapter 14 . Performance Tuning 377 To determine the value of the long_query_time variable, use the mysqladmin variables command. The results are shown in Figure 14-4. Figure 14-4: The long_query_time variable controls how long a query has to take before it is recorded in the Slow Query Log. You can change the value for long_query_time by adding the following to the MySQL configuration file under the [mysqld] section: set-variable = long_query_time= The value is given in seconds. For example, to shorten the long_query_time value to 3 seconds, you would add the following line to the [mysqld] section to the MySQL configuration file, like this: set-variable = long_query_time=3 Troubleshooting and performance functions with mysqladmin The mysqladmin command has numerous functions that assist with server administration. Cross- Reference Chapter 10 covers these functions in detail.
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376 Part III . Administration The query log records connections and queries made to the MySQL server. Using the query log, you can debug connection problems or query issues with the server. The query log is enabled by adding the logoption below the [mysqld]section in the MySQL configuration file. By default, the query log is recorded in the data directory of your MySQL server and is named .log. This can be changed by adding a =
to the log option in the configuration file, like this: log = /var/log/mysqlquery.log An example of the data stored in the query log is shown in Figure 14-3. Figure 14-3: An example of the query log in MySQL I ve used the query log to watch poorly written applications perform poorly written queries and then leave their connections open. Another log that can be used for this purpose is the slow query log, which records two types of slow queries: . Queries that take longer than the value of the long_query_time variable . Queries that don t use an index with the log-long-format option To enable the slow query log, add the log_slow_queries option underneath the [mysqld] section of the MySQL configuration file. You can also change the target for the logfile from the default of -slow.log to another name by adding = when using the log_slow_queries option, as in the following example: log_slow_queries = slowquery.log
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Chapter 14 . Performance Tuning 375 Figure 14-1: A sample error log where the server crashed Figure 14-2: An example of the binary log recorded with MySQL
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374 Part III . Administration Cross- Reference Chapter 11 examines the MySQL configuration files in detail. MySQL logfiles A feature I use frequently for troubleshooting is the MySQL logfiles. The four common logfile types are listed in Table 14-1. Table 14-1 Types of Logfiles with MySQL Log Type Use Error log Records errors encountered by the server. Query log Stores all connections and queries. Slow query log Stores queries that take longer than long_query_time or don t use indexes. Binary log Stores all updates or changes to data, used in replication Note Two logfile types, the isam and the update logs, are outside the scope of this book. One logfile of particular importance is the MySQL error log. If the server won t start, many operating systems will record a trace that can indicate why the server won t start or why it crashed. Starts and stops of the server are recorded in the error logfile. An example error log is shown in Figure 14-1. By default, the error log is stored in the data directory for MySQL with a name of .err(is the hostname of the server). To change the location of the error log, set it with the err-log option underneath the [safe_mysqld] section of the MySQL configuration file. err-log = /var/log/mysql.err To store all inserts and updates to data, use the binary log. The log-binoption under the [mysqld] section of the configuration file enables the binary log, which is stored in the data directory. The binary log is also useful during replication on the replication master, as the logfile that records updates for the slaves. This file contains binary data; the contents of a binary update log are shown in Figure 14-2.
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14 1414 CHAPTER Performance Tuning …. MySQL AB has gone to great lengths to see that MySQL performs well. However, the developers can t foresee every implementation of MySQL. It s impossible to set every variable appropriately by default. For this reason, many variables can be set by the database administrator to improve performance for the specific application. Though tips and hints for enhancing and improving the performance of the MySQL server are dispersed throughout the book, this chapter provides additional information for setting variables and improving MySQL performance Troubleshooting Servers As with the first portion of Chapter 11 (which treats enhancing the performance of the MySQL server), this first part of Chapter 14 examines troubleshooting and improving the server itself. One of the most important methods for troubleshooting server problems is to examine logfiles. I ll look at the different logfiles that are available with MySQL as the first portion of this chapter. Knowing how to determine what needs improvement is indispensable to improving server performance. Therefore I ll begin with basic troubleshooting techniques and commands, and then recap some server variables and performance- related options. MySQL AB has included some good suggestions for performance- related variables within their sample configuration files. I recommend using those files for examples to match your server configuration and resource availability. In This Chapter Troubleshooting the Server Optimizing Databases and Tables Optimizing Queries ….
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