MySQL Administrator's Bible- P12

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MySQL Administrator's Bible- P12: Với tập trung đặc biệt vào việc phát hành lớn tiếp theo của MySQL, nguồn tài nguyên này cung cấp một khuôn khổ vững chắc cho bất cứ ai mới vào MySQL hoặc chuyển từ một nền tảng cơ sở dữ liệu, cũng như các quản trị MySQL kinh nghiệm. Các bộ đôi tác giả cao hồ sơ cung cấp bảo hiểm thiết yếu của các nguyên tắc cơ bản của phạm vi bảo hiểm cơ sở dữ liệu quản lý, bao gồm cả cách tiếp cận độc đáo MySQL của các tính năng cơ sở...

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  1. Logging and Replication W hen changes are made to the database mysqld writes to binary logs, which are used in the replication process. Much IN THIS CHAPTER of this chapter concerns configuration options that are used Logging in MySQL to manage both logging and replication. These options can be configured from server startup using the command line or set in the configuration file. Configuring replication Unless the material specifically says that an option can only be configured from the command line or the configuration file, either method is possible. Replication topologies In addition, many of the configuration options can be controlled at server Correcting data runtime. Log Files Four log files are used by mysqld: the error log, the binary log, the gen- eral query log, and the slow query log. A fifth log type, the relay log, is used by a slave server. If the logs are enabled, mysqld writes them to the data directory unless otherwise specified. By default, no logging is enabled. If you find that an error log is being written to when the configuration file does not enable the error log, there is no need to worry. Some startup scripts specify error log files. These startup scripts are dependent upon the installation. Error log The error log contains entries for when the mysqld daemon started and stopped and also any critical errors that occur while the server is running. 517
  2. Part III Core MySQL Administration Information such as when the event scheduler executes an event and when replication starts and stops is also written to the error log. On some operating systems the error log additionally will write a stack trace if mysqld dies. This stack trace can be very useful for troubleshooting system failures. The error log is a plain text file. To enable the error log, specify the log-error option. The error log will be written to the data directory using the format host_name.err. To customize the filename, give the log-error option a filename, such as: log-error=/var/log/mysqld.err The log_warnings option is used to control whether or not warning messages are logged to the error log. The default value is 1 (enabled). If the value is greater than 1, aborted connections are written to the error log. Warning logging can be disabled using a value of 0. If you do not specify log-error, or if you use the --console option on Windows, errors are written to stderr, the standard error output. Even though you are not required to have an error log, it is extremely useful to help troubleshoot issues. Binary logs The binary logs are used for several reasons. They can be used to perform a point-in-time recov- ery during a recovery process. How this is done is detailed in Chapter 13. Another function of the binary log is to enable replication. This is covered later in this chapter. The contents of the binary log are any statements that occur in the server that could potentially modify the databases. Non-modifying statements such as SELECT are not logged. However, a non-modifying statement will be logged if it is part of a transaction that modifies data, because the entire transaction will be logged. To enable binary logging, use the log-bin option. The binary log index file is a plain text file that keeps track of the current binary logs. By default, its name is mysql-bin.index. To set the filename and path of the binary logs and binary log index file, specify the following options: log-bin = /data/logs/binary/changelog log-bin-index = /data/logs/relay/binarylog.index Binary log data is stored in a binary format. This means you can not just open the file with a text editor and read it. To display the binary logs in text/readable format you must use the mysqlbinlog tool. The operation of the mysqlbinlog tool is pretty straightforward. In the following example the entire contents of the mysql-bin.00001 binary log are converted into text format and copied into a new file called output.sql: shell> mysqlbinlog mysql-bin.00001 > output.sql 518
  3. Logging and Replication 16 If you leave off the > output.sql the contents will be sent to the console. Numerous options can be used with mysqlbinlog. Table 16-1 lists the frequently used mysql- binlog options. The standard options for command-line programs that interact with mysqld, such as -u, -h, -p, and -P, are not listed TABLE 16-1 Frequently used mysqlbinlog Options Option Description read-from- Used to read the binary log from a remote server. remote-server start-position Determines the position in the binary log where mysqlbinlog begins reading data. end-position Determines the position in the binary log where mysqlbinlog stops reading data. start-datetime Begins reading from the binary log at the first event with a timestamp equal to or greater than the specified datetime. end-datetime Stops reading from the binary log at the first event with a timestamp equal to or greater than the specified datetime. The following is a more complicated example, reading from a remote server: shell> mysqlbinlog --read-from-remote-server -uuser -p \ mysql-bin.000001 -h -P 3306 \ --start-position=932 --stop-position=1132 > remote000001.sql Here is an example showing how to use the time-based parameters: shell> mysqlbinlog mysql-bin.000001 \ --start-datetime="2009-02-15 17:34:40" \ --stop-datetime="2009-02-15 17:34:56" > sixteensecs.sql This is a good way to extract a smaller log if you know approximately what time an event hap- pened. mysqlbinlog does not change the original binary log. 519
  4. Part III Core MySQL Administration Relay logs Relay logs are used by a replication slave to record events received from the master server before execution on the slave server. They utilize the same binary format as the binary logs and, like the binary logs, can be viewed using the mysqlbinlog tool. By default a slave server stores the relay logs in datadir. In addition to the relay logs there is the relay-log.index file, which is used to keep track of the currently used relay log. Also, the file documents the currently used relay log file and position, plus the position in the master binary log. If you want to change the location for the relay logs, there are three configuration values you will want to add to the configuration file. These values are relay-log, relay-log-index, and relay-log-info. If you wanted to store all these logs in the directory /data/logs/relay you would need to add this to your MySQL configuration file: relay-log = /data/logs/relay/relay-bin relay-log-index = /data/logs/relay/relay-bin.index relay-log-info-file = /data/logs/relay/ As you can see, you can specify filenames in addition to directories. Relay logs are automatically deleted by the SQL thread of the slave server when it has executed all events in the log and it is no longer needed. General and slow query logs The general query and slow query log files are used to log the activities occurring in the database. If the general query log is enabled it logs all activity on the server. In addition to the actual SQL statements being executed it logs information such as when clients both connect and disconnect. It can be a very useful tool when troubleshooting issues. The general log writes statements as they are sent to mysqld, including ones that result in errors. It is not advisable to turn on the general query log on a production machine unless it is really necessary. Because it logs all server activity it can be quite a detriment to performance. The general query log can be turned on and off while the server is still running, so if a general log is needed for debugging purposes it can be enabled for a short period of time. Beginning in MySQL Server 5.1 general and slow queries can be logged to either a log file or a table in the mysql database. The general queries are logged to the general_log table and the slow queries are logged to the slow_log table. Why would you want to log this information to a database table? If all log statements are stored in a table rather than in a text file, you can use a simple SELECT statement to query the data — speeding up the information search of your logs. 520
  5. Logging and Replication 16 To set the location of both the slow and general query log output, set the server variable log_output. Possible values for log_output are: ■ FILE ■ TABLE ■ NONE ■ FILE,TABLE The default value is FILE. The log_output server variable is an example of a dynamic variable. That means that it can be set while the server is running. As an example, to set log_output to NONE (turning off both general and slow query logs), execute the following using mysql: mysql> SET GLOBAL log_output=NONE; Table 16-2 lists the server variables used to manage the general and slow query logs. TABLE 16-2 Server Variables Used to Manage General and Slow Query Logs Server Variable Description log_output= [ NONE | Determines where both general query and slow query logs are FILE | TABLE | stored. FILE,TABLE] general_log= [ON|OFF] Used to turn on or off the general query log. Possible values are ON and OFF. general_log_file Specifies the location of the general query log file. The default [=filename] location is the data directory. slow_query_log Possible values are ON and OFF. slow_query_log_file Specifies the location of the slow query log file. The default [=filename] location is the data directory. long_query_time=num Queries that take longer than num seconds are logged in the slow query log. Defaults to 10. log-queries-not- Queries that do a full table scan will be logged. using-indexes min_examined_row_ Only logs queries in slow query log that examine at least num limits=num specified rows. Defaults to 0. This option is usually used in conjunction with log-queries-not-using-indexes, to log queries on large tables that do not use indexes. 521
  6. Part III Core MySQL Administration The slow query log is similar to the general query log but only logs queries that take more than the number of seconds specified by the server variable long_query_time. The long_query_time server variable is used to control the granularity of the logging of slow queries. Up until mysqld version 5.1.21 you could only log queries that took a minimum of two seconds to execute (it would log queries greater than the minimum of one second). Beginning with version 5.1.21 you can specify the minimum query execution time in fractions of a second. For example, you could set long_query_time = .05 (fifty milliseconds). In all versions of mysqld, the long_query_time option defaults to 10 (seconds). Like the error log, in almost every situation it will make sense to have the slow query log enabled. In fact, you should determine what amount of time it takes before you consider a query slow. For a web application, 4–5 seconds is the maximum most people will wait for a regular page to load. It is quite common to see a 2-second limit or even smaller. Because it logs the time a query took, the slow query log does not write a statement until after it has finished. It will write statements that have failed, so long as they have taken more than long_query_time seconds. To enable the general_log you have to set general_log = ON and, in a similar way, to enable slow_log you have to set log_slow_queries = ON. After logging is enabled you can choose log_output but, whereas there are two separate variables for enabling the general and slow query log, there is only one variable related to log output. So you can have the general_log enabled and the slow_log disabled, but if you choose an output (and both logs are enabled) that output (FILE, TABLE, or FILE,TABLE) applies to both the logs. If the logs are disabled, no logging occurs regardless of the value of log_output. This is why no logging occurs by default, because the logs are disabled, even though the default for log_output is FILE. Rotating logs Log files can consume large amounts of disk space quickly and if they are rotated they can be compressed for archival purposes or deleted if not needed. Even if you have plenty of disk space, log files that become too large make it difficult to work with when troubleshooting and performing general monitoring. The most common methods of log file rotation are covered in the following sections. flush logs You can manually rotate the binary logs and error log by issuing a FLUSH LOGS command from mysql. When this is done the server closes the binary log currently in use and creates a new binary log by incrementing the sequence number of the file by one (in relation to the previous log). With the flushing of the error log the server: ■ Closes the current file ■ Creates a new error log ■ Renames the old error log with a suffix of -old 522
  7. Logging and Replication 16 For the other log files, FLUSH LOGS only closes and then reopens the log file. However, this means that other log files can be rotated by moving the original file, creating a blank file to replace the original, and performing a FLUSH LOGS statement. The mysladmin utility can perform a FLUSH LOGS from the shell. Here is an example of this type of usage: shell> mysqladmin flush-logs max_binlog_size The max_binlog_size option automatically rotates the binary log after it reaches a certain size. The option name is a bit of a misnomer because the binary logs are rotated after the size is reached, so the size of the file may be slightly larger than max_binlog_size. Values can be specified as integers (which indicate bytes) or using K, M, or G to specify kilobytes, megabytes, or gigabytes. For example, to set mysqld to rotate binary logs after they reach 512 MB in size: max_binlog_size=512M purge binary logs The PURGE BINARY LOGS command is another manual method of log management that is specifically designed for the binary logs. Unlike FLUSH LOGS, it deletes logs instead of saving them. There are two methods of purging: by filename and by timestamp. For example to purge all binary logs older than ’mysql-bin.00987’: mysql> PURGE BINARY LOGS TO ’mysql-bin.00987’; To purge all logs older than December 13, 2008 at 11:00 PM: mysql> PURGE BINARY LOGS BEFORE ’2008-12-13 23:00:00’; The MASTER keyword is an alias for BINARY; this command is also called PURGE MASTER LOGS. expire_logs_days The expire_logs_days option purges binary logs after a configured number of days. Some- thing to keep in mind is that if your slave servers fall behind the master it is possible to have the master delete old binary logs and ruin slave replication because it deleted data that had not been replicated to the slave server yet! Other methods of rotating With the Red Hat rpm installations of mysqld there should be a mysql-log-rotate script that can be used to rotate your log file. If you are using a non–Red Hat system you can create 523
  8. Part III Core MySQL Administration your own script and run it from cron for log rotation. Unfortunately, FLUSH statements cannot be used inside an event in mysqld. Some operating system distributions, such as Debian, include a general script for log rotation called logrotate. Some specific hooks for the MySQL logs are included with the Debian MySQL packages. If you are using Debian this will almost certainly take care of any log rotation needs you have. Various home-grown scripts are also available on the Internet and if none of these suit your pur- poses it would certainly be feasible to write you own without much difficulty. Replication Replication is the ability to keep distributed databases synchronized by copying either executed statements or data set changes from a master server to one or more slave servers. It is the pri- mary method of keeping two mysqld instances synchronized. This could be done for a number reasons including: ■ Failover requirements — high availability ■ Load balancing and scale out ■ Off-site processing — backups, running reporting queries, and so on See Chapter 22 for more information on using replication to achieve scalability and high availability. With mysqld a slave can have only a single master. However, a master can have any number of slaves. In addition, it is possible for a master server to be a slave of another master. This allows you to build some fairly complicated replication topologies. MySQL replication is asynchronous. This means that the master does not wait for slaves to acknowledge receiving the data before committing any changes. The master simply writes events to the binary log with no regard to the status of any slave servers. This means there is no guarantee that replication is actually synchronizing data appropriately. This is in contrast to semisynchronous replication, where a thread that performs a transaction commit on the master blocks after the commit is done and waits until at least one semisynchronous slave acknowledges that it has either received all events for the transaction or a timeout occurs. The benefit to this is immediately apparent — because both servers are always synchronized there is no chance of losing committed data. Even if your master server experienced a catastrophic hardware failure the slave would be guaranteed to have any committed changes. This guarantee comes at a price. Semisynchronous replication does have a performance impact because commits are slower due to the need to wait for slaves. The amount of slowdown is at least the amount of time it takes the TCP/IP packet to go from the master to the slave and back. This is needed to send the commit to the slave and wait for the acknowledgment of receipt by the slave. This is the tradeoff for increased data integrity. This means that semisynchronous replication works best for close servers communicating over fast networks. You probably should not use it for situations where servers are not on the same network. 524
  9. Logging and Replication 16 This capability was not included prior to mysqld version 6.0. Several patches are available that allow this capability in previous versions of mysqld. With mysqld version 6.0 Sun has created the capability for semisynchronous replication using a plugin-based system. This will allow for a great deal of flexibility in the future. The actual code is based on code developed at Google by Mark Callaghan and Wei Li. It was ported to version 6.0 of the server as plugin components. Though the addition of semisynchronous replication is a great option, moving replication to a plugin-based system is far more exciting. This allows a great deal of flexibility. Several plugins are already being developed, including one for delayed replication. Semisynchronous replication differs from true synchronous replication in that the master server does not wait for all slaves to acknowledge receipt of events. In addition, the master requires only acknowledgment of receipt, not that the events have been both fully executed and committed on the slave side. Synchronous replication is not available in mysqld. MySQL Cluster uses synchronous replication. In a replication setup the data is written by the master server to the binary log. Any slave servers connect to the master server using what is called an I/O thread. This process writes the statements or data received from the master into the relay log. The SQL thread is the process that reads from the relay log and then replays the statements or data changes for the MySQL process on the slave server. The end result of this process is that the slave server has executed the same statements or data changes that the master server executed. If a slave server is also configured as a master server of other slaves, it simply writes its own binary logs and the secondary slaves read from that binary log using their I/O threads. Setting up semisynchronous replication When you set up replication it is configured by default to be asynchronous. Some extra steps are involved in configuring replication to be semisynchronous. To begin using semisyncronous repli- cation the master server and one or more slave servers must be configured for semisynchronous replication. If you do not do this replication will default to asynchronous replication. In addi- tion, asynchronous replication must already be running. Semisynchronous replication is implemented using plugins. These plugins are not distributed with the server code itself and must be downloaded separately. The semisynchronous replication plugins are available at and work only on Linux. A single file contains the plugins for both master and slave servers. You need to install the libsemisync_master* files in the plugin directory of the master server and the libsemisync_slave* files in the plugin directory of each slave server. If you do not know the location of the plugin directory, the command SHOW GLOBAL VARIABLES LIKE ’plug% should tell the location of the plugin directory. The plugins are available from 525
  10. Part III Core MySQL Administration After a plugin has been installed on a server, you control it by modifying the system variables associated with it. Until a plugin is installed these system variables are not available. To load a plugin you should use the INSTALL PLUGIN statement on the master and on each slave that is to be configured with semisynchronous replication. On the master: mysql> INSTALL PLUGIN rpl_semi_sync_master SONAME ’’; On each slave: mysql> INSTALL PLUGIN rpl_semi_sync_slave SONAME ’’; The preceding commands use a plugin filename suffix of .so. A different suffix might apply on your system. If you are not sure about the plugin filename, look for the plugins in the server’s plugin directory. Two simple methods exist for determining which plugins are installed on a running server: ■ Use the SHOW PLUGINS statement ■ Query the INFORMATION_SCHEMA.PLUGINS table Installing the semisynchronous replication plugin will enable it by default. To use semisyn- chronous replication you must have the plugins enabled both on the master server and at least one slave server. If only one side is enabled any replication that occurs will be asynchronous. To control an installed plugin you must set the appropriate system variables. You can set these variables at runtime using SET GLOBAL, or at server startup on the command line or in an option file. At runtime, these master server system variables are available: mysql> SET GLOBAL rpl_semi_sync_master_enabled = {0|1}; mysql> SET GLOBAL rpl_semi_sync_master_timeout = N; On the slave side, this system variable is available: mysql> SET GLOBAL rpl_semi_sync_slave_enabled = {0|1}; For rpl_semi_sync_master_enabled or rpl_semi_sync_slave_enabled, the value should be 1 to enable semisynchronous replication or 0 to disable it. By default, these variables are set to 1. For rpl_semi_sync_master_timeout, the value N is given in seconds. The default value is 10. 526
  11. Logging and Replication 16 If you enable semisynchronous replication on a slave at runtime, you must also start the slave I/O thread (stopping it first if it is already running) to cause the slave to connect to the master and register as a semisynchronous slave: mysql> STOP SLAVE IO_THREAD; mysql> START SLAVE IO_THREAD; If the I/O thread is already running and you do not restart it, the slave continues to use asyn- chronous replication. At server startup, the variables that control semisynchronous replication can be set as command-line options or in an option file. Typically, you set the variables in the my.cnf or my.ini files on the master and slave servers. On the master: [mysqld] rpl_semi_sync_master_enabled=1 rpl_semi_sync_master_timeout=10 On each slave: [mysqld] rpl_semi_sync_slave_enabled=1 At this point you how have semisynchronous replication enabled and running. Statement-based, row-based, and mixed-based replication You have three methods of logging the data for replication: ■ Statement-based replication (SBR) replicates SQL statements that modify data. ■ Row-based replication (RBR) replicates only the changed rows. ■ Mixed-based replication (MBR) is comprised of a combination of statement-based replica- tion and row-based replication. Currently for all versions of mysqld, statement-based replication is the default method of logging. Most storage engines support all three methods of logging data. The two exceptions are that Blackhole does not support row-based replication and Falcon does not support statement-based replication. If mixed-based replication is in use it will utilize statement-based replication by default but will switch to row-based replication when needed. The reasons that RBR will be used are the following: ■ When a function is called that contains the UUID() function. ■ When two or more tables containing AUTO_INCREMENT fields are updated. 527
  12. Part III Core MySQL Administration ■ Upon execution of the INSERT DELAYED statement. ■ When a UDF is called. ■ When the following functions are used: ■ FOUND_ROWS() ■ ROW_COUNT ■ USER() ■ CURRENT_USER() ■ CURRENT_USER The use of row-based replication definitely resolves some issues with statement-based replica- tion that are difficult, if not impossible, to fix otherwise. However, the relative newness of RBR means that it is not as widely deployed and is not as well understood by MySQL administrators. As well, a very small statement can change a very large amount of data, so RBR may be more bandwidth-intensive than SBR. The mysqlbinlog tool has been updated to support RBR, but the output can still be somewhat difficult to decipher. If you just run the mysqlbinlog tool without special options shell> mysqlbinlog mysql-bin.00001 # at 238 #081213 13:14:29 server id 1 end_log_pos 1278 Write_rows: table id 17 flags: STMT_END_F BINLOG ’ fAS3SBMBAAAALAAAANoAAAAAABEAAAAAAAAABHRlc3QAAXQAAwMPCgIUAAQ= fAS3SBcBAAAAKAAAAAIBAAAQABEAAAAAAAEAA//8AQAAAAVhcHBsZQ== ’/*!*/; you will have binary data where the row-based events are in the log. Using the –v (verbose) option will enable to you to view the statements that made the data changes: shell> mysqlbinlog –v mysql-bin.00001 # at 238 #081213 13:14:29 server id 1 end_log_pos 1278 Write_rows: table id 17 flags: STMT_END_F BINLOG ’ fAS3SBMBAAAALAAAANoAAAAAABEAAAAAAAAABHRlc3QAAXQAAwMPCgIUAAQ= fAS3SBcBAAAAKAAAAAIBAAAQABEAAAAAAAEAA//8AQAAAAVhcHBsZQ== ’/*!*/; ### INSERT INTO test.a ### SET ### @1=1 ### @2=’testing’ 528
  13. Logging and Replication 16 ### @3=NULL ... # at 1298 #081213 13:14:29 server id 1 end_log_pos 1326 Update_rows: table id 17 flags: STMT_END_F Your output will now contain pseudo SQL statements that are commented out that represent the row-based events. This output is much better than the default, but if you look closely you will see that the INSERT statement does not include field names — only placeholders (@1, @2, @3). To be clear, the necessary data for replication and point-in-time recovery is stored in the binary log, but it just is not as easy to decipher as with statement-based replication logs. Replication Configurations Replication setup can be as simple as a master and a slave server. It can be as complex as what is called circular replication, where the servers form a ring with each master also being a slave of another master. Simple replication The simplest replication setup is a single master with a single slave. We will use this example to show the basics of replication setup. Figure 16-1 shows a simple master/slave configuration. FIGURE 16-1 Master Slave To begin with you need to edit the configuration file on both servers. On the master server in the [mysqld] directive you need to add: server-id= X log-bin = mysql-bin sync_binlog= 1 529
  14. Part III Core MySQL Administration The server-id value of X needs to be a positive integer that is unique among all your database servers. Often administrators will use the last octet of the server’s IP address because this will typically be unique from server to server. For example, if the server’s IP address was you would use 100 as the server-id. To use more unique integers, the serial number of the system or the entire IP address could be used, such as 192168002100, or even the IP address plus the port, such as 1921680021003306. If you do not use unique values of server-id among your replication servers unpredictable results will occur. Quite often if two slaves have the same server-id a master will send some data to one slave, then send different data to another slave, leaving both with an incomplete set of data changes. After the configuration changes are made you must restart mysqld. You must restart mysqld anytime changes are made to the configuration file for the changes to take effect. Once this is done you must create a replication user on the master server. To do this, log in to mysqld and issue the following command: mysql> CREATE USER ’username’@’host’ IDENTIFIED BY ’password’; mysql> GRANT REPLICATION SLAVE ON *.* -> TO ’user’@’host’ IDENTIFIED BY ’password’; You will need to replace host with the hostname or IP address of the slave server. At this point you are done on the master server setup. You must now edit the configuration file on the slave server. In the [mysqld] section you need to add: server-id = X As before, for the server-id line, X should to be a unique positive integer that is different than any other server-id entries on other database servers. Most of the configuration on a slave server is done dynamically using the CHANGE MASTER command. After this change is made you need to restart mysqld on the slave. It is important to understand that in order to utilize replication you must have the same data sets on both servers. If your master server has no data this is simple; if this is not the case it is more difficult. You have multiple ways of ensuring you have the same data set on both servers. We cover the details of backing up and restoring in Chapter 13. The basic idea is that you need to take a consistent backup of the master, noting down at what point in the master binary logs the backup was taken. The --master-data option to mysqldump will automatically store a CHANGE MASTER TO statement at the end of the export. On import, the slave information will already be set up. In a physical backup, copying the file is all that is needed. 530
  15. Logging and Replication 16 After the backup is restored on the slave, log in and execute a START SLAVE command and then the SHOW SLAVE STATUS command: mysql> START SLAVE; mysql> SHOW SLAVE STATUS; *************************** 1. row *************************** Slave_IO_State: Waiting for master to send event Master_Host: Master_User: replica Master_Port: 3306 Connect_Retry: 60 Master_Log_File: mysql-bin.000001 Read_Master_Log_Pos: 548 Relay_Log_File: mysql-relay-bin.000001 Relay_Log_Pos: 600 Relay_Master_Log_File: mysql-bin.000001 Slave_IO_Running: Yes Slave_SQL_Running: Yes Replicate_Do_DB: Replicate_Ignore_DB: Replicate_Do_Table: Replicate_Ignore_Table: Replicate_Wild_Do_Table: Replicate_Wild_Ignore_Table: Last_Errno: 0 Last_Error: Skip_Counter: 0 Exec_Master_Log_Pos: 4 Relay_Log_Space: 3630 Until_Condition: None Until_Log_File: Until_Log_Pos: 0 Master_SSL_Allowed: No Master_SSL_CA_File: Master_SSL_CA_Path: Master_SSL_Cert: Master_SSL_Cipher: Master_SSL_Key: Seconds_Behind_Master: 0 Master_SSL_Verify_Server_Cert: No Last_IO_Errno: 0 Last_IO_Error: Last_SQL_Errno: 0 Last_SQL_Error: 1 row in set (0.00 sec) Both Slave_IO_Running and Slave_SQL_Running must show YES for the slave to be run- ning correctly. Table 16-3 lists each field from the SHOW SLAVE STATUS command and a brief description. 531
  16. Part III Core MySQL Administration TABLE 16-3 SHOW SLAVE STATUS Field Descriptions Field Name Description Master_Host The hostname or IP address of the master server. Master_User User used to establish the connection to the master host. Master_Port The TCP/IP port to connect to on the master host. Connect_Retry The amount of time that the slave waits before attempting to reconnect to the master server after a connection failure. The default value is 60 (seconds). Master_Log_File The master binary log file that the slave I/O thread is reading from. Read_Master_Log_Pos The position in the master binary log that the I/O thread is currently reading from. Relay_Log_File The relay log file on the slave from which the SQL thread is reading. Relay_Log_Pos The current position of the SQL thread in the relay log. Slave_IO_Running If this has a value of YES the I/O thread has started and connected successfully to the master host. If NO it has stopped running; for example, if the network connection is lost. Slave_SQL_Running If this has a value of YES the SQL thread is running. If it has a value of NO the SQL thread is not running. A statement causing an error can cause the SQL thread to stop running. Replicate_Do_DB Specifies a database which will be replicated. There can multiple entries for Replicate_Do_DB in a comma-separated list. Replicate_Ignore_DB Specifies a database that will not be replicated. There can be multiple entries for Replicate_Ignore_DB in a comma-separated list. Replicate_Do_Table Specifies a table that will be replicated. There can be multiple entries for Replicate_Do_Table in a comma-separated list. Replicate_ Specifies a table that will not be replicated. There can be Ignore_Table multiple entries for Replicate_Ignore_Table in a comma-separated list. Replicate_Wild_ A ‘‘wildcard’’ entry specifying databases that will be Do_Table replicated. 532
  17. Logging and Replication 16 TABLE 16-3 (continued ) Field Name Description Replicated_Wild_ A ‘‘wildcard’’ entry specifying databases that will not be Ignore_Table replicated. Last_Errno The error number of the last replication error that occurred. Last_Error A description of the last replication error that occurred. This may include an SQL statement that caused the SQL thread to stop. Skip_Counter The last value of num used when executing SET GLOBAL SQL_SLAVE_SKIP_COUNTER=num. Exec_Master_Log_Pos The position of the last event executed by the SQL thread from the master binary log. Relay_Log_Space The total size of all relay logs. Seconds_Behind_Master Shows how far the SQL thread is behind the IO thread in execution of the relay logs. This is calculated by taking the last timestamp value in the relay log and subtracting the timestamp value for the current event that the SQL thread is executing. Seconds_Behind_Master does not give any indication of how long the slave will take to become up-to-date. If the IO thread is not able to keep the relay logs current with the master binary logs, Seconds_Behind_Master can be falsely reassuring. If the IO thread cannot keep up with the volume of master binary logs, but the SQL thread can keep up with the IO thread, the value of Seconds_Behind_Master can be 0 even though the slave is lagging — potentially far behind. Master_SSL_Verify_ Possible values are 0 (do not verify that the Common Name Server_Cert of the server certificate matches the hostname that the client connected to) or 1 (perform the verification, valid only when encrypted connections are in use). The default value is 0. Last_IO_Errno The last error number encountered by the IO thread. Last_IO_Error A description of the last error encountered by the IO thread. Last_SQL_Errno The last error number encountered by the SQL thread. Last_SQL_Error A description of the last error encountered by the SQL thread. 533
  18. Part III Core MySQL Administration CHANGE MASTER statement If your master server information was not stored in the backup or if you are configuring a slave of a master server with an empty data set, you will need to configure the connection informa- tion using the CHANGE MASTER command. The CHANGE MASTER command can also be used to change the slave information after the first slave configuration; for example, when you are pro- moting a slave to be a new master and want to change other slaves to stop replicating the old master and start replicating the new master (see Chapter 22 for examples of slave promotion). To do this, log in to mysqld, and execute the following command while replication is stopped: CHANGE MASTER TO MASTER_HOST=’host_name’, MASTER_USER=’user’, MASTER_PASSWORD=’password’, MASTER_LOG_FILE=’mysql-bin.000001’, MASTER_LOG_POS=98; The five fields listed here are the minimum data that mysqld needs to be able to configure replication. Almost all of the fields listed in Table 16-3 can be used in the CHANGE MASTER TO statement; for example, MASTER_PORT and MASTER_CONNECT_RETRY can be set. Once this has been executed, you can then execute a START SLAVE statement and finally the SHOW SLAVE STATUS command. If replication is not working smoothly, check the error log for more information. More complex setups It is possible to build more complicated replication setups than the simple master-slave setup. Each of the following sections briefly describes an elaborate setup you can use as needed. Single master and multiple slaves You can set up as many slaves as needed. Figure 16-2 illustrates a master server with data changes being replicated to three slaves. FIGURE 16-2 Master Slave Slave Slave 534
  19. Logging and Replication 16 One common reason for a setup like this is to help alleviate the load on the master database. The application can read from any slave and receive data that is frequently as up-to-date as needed (replication is asynchronous by default, so the data may be out of date). There is no difference in the previous configurations discussed for the master or the additional slave servers in this setup. As pointed out before, each server must have a unique server-id entry in the [mysqld] directive of the configuration file. You should understand when working with multiple slaves that each slave causes a small amount of overhead on the master server. Even on busy servers this is not very much until you begin adding a large number of slaves directly to a master. Master and relay slave If you need more slaves than a master can comfortably manage, it is time to introduce a relay server. The sole function of this server is to relay the binary logs from the master to all the slaves. Figure 16-3 shows this type of setup. FIGURE 16-3 Master Relay Slave Slave Slave Slave Slave Slave Slave This will reduce the load on the master server. The configuration for this setup will need to be slightly different than before. The relay slave will need a specific configuration. In the [mysqld] section you will need to have: server-id = X log-bin = mysql-bin log-slave-updates 535
  20. Part III Core MySQL Administration The difference between this configuration and the standard configuration is the addition of the log-slave-updates and log-bin statements. This tells the server that it should write binary logs and include the changes it receives from the master. Otherwise this cascading replication configuration will not work. The six slaves on the bottom level of our example do not need to have log-slave-updates or log-bin configured. There are other reasons to enable the binary logs, including incremental backup, point-in-time recovery, and consistent configuration setups in the event that one of the slaves on the bottom is promoted to be a relay slave. Though not necessary, it can be useful to use the Blackhole storage engine on the relay slave. With the Blackhole storage engine the data is not actually stored in a table. All changes are written to the binary logs, which then enable the changes to propagate to the lower-level slaves. Because tables are not actually maintained the I/O demands on the server are typically much less. This will improve the overall performance of the server. You can find more details on the Blackhole storage engine in Chapter 11. A final note about this: you could of course add another level of slaves below these six slaves if needed. As with the relay slave in the example you would have to make sure the log-slave-updates and log-bin statements are in the configuration file for the slaves that are replicating to other slaves. Master-Master replication A common way of providing a form of high availability is through the use of what is called master-master replication. With this topology there are two servers, each both a master and a slave of each other. Figure 16-4 shows this setup. FIGURE 16-4 Server A Server B This setup might seem simple enough, but actually introduces some subtle issues that the database administrator needs to be aware of when working with master-master replication. 536
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