MySQL master-slave replication latency can be solved by locating causes and targeted optimization. 1. First determine the degree of delay, view the Seconds_Behind_Master value through SHOW SLAVE STATUS, and analyze the real delay with tools such as pt-heartbeat; 2. For excessive write pressure, you can upgrade the hardware, enable parallel replication or split the slave library; 3. Avoid long transactions and slow queries, and optimize the execution efficiency of master-slave SQL; 4. Check the network status, reduce binlog content and enable compressed transmission; 5. Use multi-threaded replication in MySQL 5.7, and enable parallel replication based on logical clocks in 8.0 to improve throughput capabilities; 6. Appropriately tune the relay log parameters, regularly rebuild slave libraries, and use semi-synchronous replication to enhance data security.

MySQL master-slave replication delay is a common problem in many database operations and maintenance, especially in scenarios where data volume is large and frequent writes are more likely to occur. If the delay is too high, it may affect the effect of read and write separation and even lead to inconsistent data. To solve this problem, we need to locate the cause first and then targeted optimization.

1. Determine the degree and source of the master-slave replication delay

First, you need to confirm how big the current copy delay is. The value of Seconds_Behind_Master can be viewed through SHOW SLAVE STATUS\G . This value represents the time (seconds) behind the slave library than the master library, but it does not fully reflect the real data delay. For example, when the slave library is executing a time-consuming statement, this value may be temporarily 0, but in fact, many events have been backlogged and have not been processed yet.

A more accurate way is to combine log analysis tools or monitoring systems to view the replication backlog. In addition, tools like pt-heartbeat can be used to continuously detect real replication delays.

If you find that the delay fluctuates frequently or suddenly surges, it is likely caused by certain operations, such as large batch updates, DDL operations, or insufficient slave library performance.

2. Common causes and coping methods for replication delays

Excessive writing pressure

If the write volume of the main library is too large and the slave library cannot process it, it will generate backlog. You can consider this at this time:

• Upgrade the library hardware configuration, especially disk IO and CPU capabilities
• Using Parallel Replication, MySQL 5.7 supports database-level parallel replication, and 8.0 also supports transaction group-based parallel replication
• If the business allows, split the library and distribute the pressure

Long transactions or slow query

A long transaction in the main library may also be executed on the slave library for a long time, causing copy accumulation. This situation suggests:

• Avoid executing large transactions in the main library, especially large data volumes of UPDATE or DELETE
• Regularly check the main library slow query logs and optimize slow SQL
• Turn on the slow query log from the library and observe whether the statements executed by the copy thread themselves are slowing down

Network latency or bandwidth limitations

Network instability between master and slave can also lead to delays in binlog transmission. Can:

• Monitor network status between master and slave
• Reduce unnecessary binlog content, such as setting binlog_format=ROW to avoid recording unnecessary column changes
• Enable compressed transfer (supported by MySQL 8.0)

3. Use parallel replication to improve the throughput capability of the slave library

MySQL 5.7 has begun to support multithreaded replication (MTS), and the default is to perform parallel playback by database dimension. If your business has multiple databases, this method works well; but if there is only one database, this parallel method is useless.

MySQL 8.0 introduces parallel replication based on logical clocks, which can implement parallel application transactions in the same database, greatly improving replication efficiency. To enable it:

• Set slave_parallel_type=LOGICAL_CLOCK
• Set slave_parallel_workers=N (N is generally set to the number of CPU cores)

It should be noted that although parallel replication can increase the speed, it may also bring some conflict risks, especially in scenarios with high concurrent writes. Therefore, it is best to cooperate with GTID to ensure consistency.

4. Other practical optimization suggestions

• Appropriately increase the relay log buffer size : By adjusting relay_log_buffer_size , the number of I/O times can be reduced and the replication efficiency can be improved.
• Set sync_relay_log reasonably : In order to improve performance, sync_relay_log=0 can be set, but this will increase the risk of losing relay logs due to power outage.
• Regularly rebuild slave libraries : If various fragmentation or indexing problems occur after long-term operation of the slave library, it may cause slow replication. You can consider rebuilding the slave library regularly with backup.
• Using semi-synchronous replication : Although it cannot directly reduce latency, it can prevent the slave database from being completely received after the main library commits a transaction, improving data security.

Basically these are more common and effective practices. Replication delay is not a big problem, but if not processed in time, it may affect the stability of the entire system. The key is to judge the bottleneck based on the actual situation and then prescribe the right medicine.

The above is the detailed content of Diagnosing and Reducing Replication Lag in MySQL. For more information, please follow other related articles on the PHP Chinese website!