search
HomeDatabaseMysql Tutorial如何检查MySQL数据库的主从延时?

MySQL 数据库 主从延时 如何去判断呢?本文我们介绍了两种判断方法:1. Seconds_Behind_Master vs 2. mk-heartbeat,接下来我们就分别介绍这些内容。 日常工作中,对于MySQL主从复制检查,一方面我们要保证复制的整体结构是否正常,另一方面需要检查主从数据

  MySQL数据库主从延时如何去判断呢?本文我们介绍了两种判断方法:1. Seconds_Behind_Master vs 2. mk-heartbeat,接下来我们就分别介绍这些内容。

  日常工作中,对于MySQL主从复制检查,一方面我们要保证复制的整体结构是否正常,另一方面需要检查主从数据是否保持一致。对于前者我们可以通过监控复制线程是否工作正常以及主从延时是否在容忍范围内,对于后者则可以通过分别校验主从表中数据的md5码是否一致,来保证数据一致,可以使用Maatkit工具包中的mk-table- checksum工具去检查。

  方法1:

  通过监控show slave status\G命令输出的Seconds_Behind_Master参数的值来判断,是否有发生主从延时。其值有这么几种:

  NULL — 表示io_thread或是sql_thread有任何一个发生故障,也就是该线程的Running状态是No,而非Yes。

  0 — 该值为零,是我们极为渴望看到的情况,表示主从复制良好,可以认为lag不存在。

  正值 — 表示主从已经出现延时,数字越大表示从库落后主库越多。

  负值 — 几乎很少见,我只是听一些资深的DBA说见过,其实,这是一个BUG值,该参数是不支持负值的,也就是不应该出现。

  show slave status\G,该命令的输出结果非常丰厚,给我们的监控提供了很多有意义的参数,比如:Slave_IO_Running该参数可作为 io_thread的监控项,Yes表示io_thread的和主库连接正常并能实施复制工作,No则说明与主库通讯异常,多数情况是由主从间网络引起的问题;Slave_SQL_Running该参数代表sql_thread是否正常,具体就是语句是否执行通过,常会遇到主键重复或是某个表不存在。下面就说到今天的重点Seconds_Behind_Master,该值作为判断主从延时的指标,那么它又是怎么得到这个值的呢,同时,它为什么又受到很多人的质疑?

  Seconds_Behind_Master是通过比较sql_thread执行的event的timestamp和 io_thread复制好的event的timestamp(简写为ts)进行比较,而得到的这么一个差值。我们都知道的relay-log和主库的 bin-log里面的内容完全一样,在记录sql语句的同时会被记录上当时的ts,所以比较参考的值来自于binlog,其实主从没有必要与NTP进行同步,也就是说无需保证主从时钟的一致。

  你也会发现,其实比较真正是发生在io_thread与sql_thread之间,而io_thread才真正与主库有关联,于是,问题就出来了,当主库I/O负载很大或是网络阻塞,io_thread不能及时复制binlog(没有中断,也在复制),而 sql_thread一直都能跟上io_thread的脚本,这时Seconds_Behind_Master的值是0,也就是我们认为的无延时,但是,实际上不是,你懂得。这也就是为什么大家要批判用这个参数来监控数据库是否发生延时不准的原因,但是这个值并不是总是不准,如果当io_thread与 master网络很好的情况下,那么该值也是很有价值的。

  之前,提到Seconds_Behind_Master这个参数会有负值出现,我们已经知道该值是io_thread的最近跟新的ts与sql_thread执行到的ts差值,前者始终是大于后者的,唯一的肯能就是某个event的ts发生了错误,比之前的小了,那么当这种情况发生时,负值出现就成为可能。

  方法2:

  mk-heartbeat,Maatkit万能工具包中的一个工具,被认为可以准确判断复制延时的方法。

  mk-heartbeat的实现也是借助timestmp的比较实现的,它首先需要保证主从服务器必须要保持一致,通过与相同的一个NTP server同步时钟。它需要在主库上创建一个heartbeat的表,里面至少有id与ts两个字段,id为server_id,ts就是当前的时间戳 now(),该结构也会被复制到从库上。

  表建好以后,会在主库上以后台进程的模式去执行一行更新操作的命令,定期去向表中的插入数据,这个周期默认为1 秒,同时从库也会在后台执行一个监控命令,与主库保持一致的周期去比较,复制过来记录的ts值与主库上的同一条ts值,差值为0表示无延时,差值越大表示延时的秒数越多。

  我们都知道复制是异步的ts不肯完全一致,所以该工具允许半秒的差距,在这之内的差异都可忽略认为无延时。这个工具就是通过实打实的复制,巧妙的借用timestamp来检查延时,非常好用!

  关于检查MySQL数据库的主从延时的两种方法就介绍到这里了,希望本次的介绍能够对您有所收获!

Statement
The content of this article is voluntarily contributed by netizens, and the copyright belongs to the original author. This site does not assume corresponding legal responsibility. If you find any content suspected of plagiarism or infringement, please contact admin@php.cn
What are stored procedures in MySQL?What are stored procedures in MySQL?May 01, 2025 am 12:27 AM

Stored procedures are precompiled SQL statements in MySQL for improving performance and simplifying complex operations. 1. Improve performance: After the first compilation, subsequent calls do not need to be recompiled. 2. Improve security: Restrict data table access through permission control. 3. Simplify complex operations: combine multiple SQL statements to simplify application layer logic.

How does query caching work in MySQL?How does query caching work in MySQL?May 01, 2025 am 12:26 AM

The working principle of MySQL query cache is to store the results of SELECT query, and when the same query is executed again, the cached results are directly returned. 1) Query cache improves database reading performance and finds cached results through hash values. 2) Simple configuration, set query_cache_type and query_cache_size in MySQL configuration file. 3) Use the SQL_NO_CACHE keyword to disable the cache of specific queries. 4) In high-frequency update environments, query cache may cause performance bottlenecks and needs to be optimized for use through monitoring and adjustment of parameters.

What are the advantages of using MySQL over other relational databases?What are the advantages of using MySQL over other relational databases?May 01, 2025 am 12:18 AM

The reasons why MySQL is widely used in various projects include: 1. High performance and scalability, supporting multiple storage engines; 2. Easy to use and maintain, simple configuration and rich tools; 3. Rich ecosystem, attracting a large number of community and third-party tool support; 4. Cross-platform support, suitable for multiple operating systems.

How do you handle database upgrades in MySQL?How do you handle database upgrades in MySQL?Apr 30, 2025 am 12:28 AM

The steps for upgrading MySQL database include: 1. Backup the database, 2. Stop the current MySQL service, 3. Install the new version of MySQL, 4. Start the new version of MySQL service, 5. Recover the database. Compatibility issues are required during the upgrade process, and advanced tools such as PerconaToolkit can be used for testing and optimization.

What are the different backup strategies you can use for MySQL?What are the different backup strategies you can use for MySQL?Apr 30, 2025 am 12:28 AM

MySQL backup policies include logical backup, physical backup, incremental backup, replication-based backup, and cloud backup. 1. Logical backup uses mysqldump to export database structure and data, which is suitable for small databases and version migrations. 2. Physical backups are fast and comprehensive by copying data files, but require database consistency. 3. Incremental backup uses binary logging to record changes, which is suitable for large databases. 4. Replication-based backup reduces the impact on the production system by backing up from the server. 5. Cloud backups such as AmazonRDS provide automation solutions, but costs and control need to be considered. When selecting a policy, database size, downtime tolerance, recovery time, and recovery point goals should be considered.

What is MySQL clustering?What is MySQL clustering?Apr 30, 2025 am 12:28 AM

MySQLclusteringenhancesdatabaserobustnessandscalabilitybydistributingdataacrossmultiplenodes.ItusestheNDBenginefordatareplicationandfaulttolerance,ensuringhighavailability.Setupinvolvesconfiguringmanagement,data,andSQLnodes,withcarefulmonitoringandpe

How do you optimize database schema design for performance in MySQL?How do you optimize database schema design for performance in MySQL?Apr 30, 2025 am 12:27 AM

Optimizing database schema design in MySQL can improve performance through the following steps: 1. Index optimization: Create indexes on common query columns, balancing the overhead of query and inserting updates. 2. Table structure optimization: Reduce data redundancy through normalization or anti-normalization and improve access efficiency. 3. Data type selection: Use appropriate data types, such as INT instead of VARCHAR, to reduce storage space. 4. Partitioning and sub-table: For large data volumes, use partitioning and sub-table to disperse data to improve query and maintenance efficiency.

How can you optimize MySQL performance?How can you optimize MySQL performance?Apr 30, 2025 am 12:26 AM

TooptimizeMySQLperformance,followthesesteps:1)Implementproperindexingtospeedupqueries,2)UseEXPLAINtoanalyzeandoptimizequeryperformance,3)Adjustserverconfigurationsettingslikeinnodb_buffer_pool_sizeandmax_connections,4)Usepartitioningforlargetablestoi

See all articles

Hot AI Tools

Undresser.AI Undress

Undresser.AI Undress

AI-powered app for creating realistic nude photos

AI Clothes Remover

AI Clothes Remover

Online AI tool for removing clothes from photos.

Undress AI Tool

Undress AI Tool

Undress images for free

Clothoff.io

Clothoff.io

AI clothes remover

Video Face Swap

Video Face Swap

Swap faces in any video effortlessly with our completely free AI face swap tool!

Hot Tools

SublimeText3 Chinese version

SublimeText3 Chinese version

Chinese version, very easy to use

Zend Studio 13.0.1

Zend Studio 13.0.1

Powerful PHP integrated development environment

PhpStorm Mac version

PhpStorm Mac version

The latest (2018.2.1) professional PHP integrated development tool

Dreamweaver CS6

Dreamweaver CS6

Visual web development tools

SecLists

SecLists

SecLists is the ultimate security tester's companion. It is a collection of various types of lists that are frequently used during security assessments, all in one place. SecLists helps make security testing more efficient and productive by conveniently providing all the lists a security tester might need. List types include usernames, passwords, URLs, fuzzing payloads, sensitive data patterns, web shells, and more. The tester can simply pull this repository onto a new test machine and he will have access to every type of list he needs.