How to optimize Replication lag when migrating MySQL to CloudSQL using Cloud DMS ?

Introduction:

Database migration is a critical operation for organizations looking to modernize their infrastructure, consolidate resources, or improve performance. However, ensuring minimal downtime and maintaining data consistency between the source and target databases during migration can be challenging, especially when dealing with replication lag. In this blog post, we’ll explore best practices to reduce replication lag between the source and external replica during a database migration.

Understanding Replication Lag:

Replication lag can occur due to various reasons such as network latency, heavy workload on the replica server, large transactions on the master, or inadequate hardware resources. It’s essential to monitor and understand the factors contributing to replication lag before implementing any solutions.

1. Optimize Network Connectivity:

• Ensure that the network connection between the source and external replica is robust and has sufficient bandwidth to handle replication traffic efficiently.
• Minimize network latency by using high-speed, low-latency network connections, such as dedicated network links or VPN tunnels.

2. Allocate Adequate Resources:

• Provision sufficient CPU, memory, and disk resources for the external replica to handle the replication workload effectively.
• Monitor resource utilization on both the source and external replica servers and scale up resources as needed to prevent bottlenecks.

3. Enable Binary Logging Compression:

• Enable binary log compression on the source database server to reduce the size of binary log files generated during replication.
• Compressed binary logs consume less network bandwidth during replication, thereby reducing replication lag.

4. Tune Replica Configuration Parameters:

• Adjust replica configuration parameters to optimize replication performance, such as increasing the value of `slave_parallel_workers` to enable parallel replication threads.
• Increase innodb_buffer_pool_size to improve data retrieval performance.

In Cloud SQL, the default, minimum allowable, and maximum allowable values of the innodb_buffer_pool_size flag depend on the instance’s memory. These values can be roughly calculated as a percentage of the instance’s RAM. By default, the value of this flag is typically set close to the maximum allowable value. The maximum allowable allocation percentage increases with instance size. The minimum allowable value is usually about 20% of the instance’s RAM.

• Adjust innodb_io_capacity and innodb_io_capacity_max to optimize I/O performance.
• For higher performance for read replica, set innodb_flush_log_at_trx_commit value to 2. If you set the flag value to 2, you must either disable the binary log on the replica, or set syc_binlog to a value other than 1.

Note: For full ACID compliance, and to maintain durability and consistency in a replication setup, the innodb_flush_log_at_trx_commit and the sync_binlog flags must be set to the default value of 1. Hence after promotion, these flags will be removed automatically.

5. Minimize Long-Running Transactions:

• Minimize the duration of long-running transactions on the source database to reduce replication lag on the external replica.
• Break down large transactions into smaller, more manageable chunks to avoid overwhelming the replication process.
• Monitor and optimize slow queries that contribute to long-running transactions.

6. Monitor Replication Lag:

• Monitor replication lag between the source and external replica using MySQL’s built-in replication monitoring tools (`SHOW SLAVE STATUS`, `PERFORMANCE_SCHEMA`).
• Monitor the “Replication delay” on the Cloud DMS console

• Monitor the “Replication lag” chart on the destination CloudSQL instance overview panel.

• Set up alerts or notifications to be alerted when replication lag exceeds predefined thresholds, allowing you to take timely action to address any issues.

7. Optimize Database Schema Changes:

• Minimize schema changes (DDL statements) during the database migration process to reduce the replication workload on the external replica.
• If schema changes are unavoidable, schedule them during off-peak hours to minimize their impact on replication lag.

Conclusion:

Reducing replication lag between the source and external replica is crucial for ensuring a smooth and efficient database migration process with minimal downtime. By understanding the factors contributing to replication lag and implementing appropriate solutions, you can ensure the reliability and performance of your MySQL replication setup. Remember to continuously monitor replication lag and adjust your strategies as needed to maintain optimal performance.