How to Configure the Number of Background InnoDB I/O Threads in MySQL 8 for Performance?


In MySQL, configuring the number of background InnoDB I/O threads is crucial for performance optimization, especially on servers with multiple cores and high I/O capacity. InnoDB uses background threads for tasks like flushing dirty pages, performing insert buffer merge operations, and purging delete-marked records. The key variables related to this in MySQL 8 are:

  1. innodb_read_io_threads: Controls the number of threads used for read operations.
  2. innodb_write_io_threads: Controls the number of threads used for write operations.
  3. innodb_purge_threads: Controls the number of threads used for purging operations.
  4. innodb_page_cleaners: Controls the number of threads used for cleaning dirty pages before flushing.

Runbook to Configure Background InnoDB I/O Threads

  1. Assess Your Workload: Understand your server's workload. If your workload is read or write-intensive, or if you frequently deal with large transactions, adjusting these parameters can be beneficial.
  2. Monitor Current Performance: Use tools like SHOW ENGINE INNODB STATUS to monitor current I/O patterns and identify bottlenecks.
  3. Adjusting Thread Count:
    • For Read and Write I/O Threads:
      • The default value is typically adequate for most systems (generally set to 4).
      • Increase the number in high I/O scenarios or with SSD storage.
      • Configure using: SET GLOBAL innodb_read_io_threads = X; and SET GLOBAL innodb_write_io_threads = X; where X is the desired number of threads.
    • For Purge Threads:
      • The default is 1, but on a busy system with lots of delete operations, increasing this can improve performance.
      • Configure using: SET GLOBAL innodb_purge_threads = X;.
    • For Page Cleaners:
      • The default is 1, which is sufficient for most cases.
      • For systems with a high rate of dirty page creation, increasing this can be beneficial.
      • Configure using: SET GLOBAL innodb_page_cleaners = X;.
  4. Consider System Resources:
    • Ensure your server has enough CPU cores to handle the increased number of threads.
    • Avoid setting the thread count too high as it can lead to increased contention and CPU overhead.
  5. Apply Changes Persistently:
    • Modify your MySQL configuration file (my.cnf or my.ini) to make these changes persistent across restarts:
  1. Test and Validate:
    • After making changes, monitor your system's performance to ensure that the changes have a positive impact.
    • Look for reduced I/O wait times and improved throughput.
  2. Adjust as Needed:
    • Depending on the results, you might need to fine-tune these settings further.


Proper configuration of InnoDB I/O threads can significantly impact the performance of your MySQL database, particularly under high load conditions. It's important to base your configuration on the specific needs of your workload and the capacity of your hardware. Always monitor the impact of any changes and adjust as necessary. Read more about InnoDB optimization here.

About Shiv Iyer 456 Articles
Open Source Database Systems Engineer with a deep understanding of Optimizer Internals, Performance Engineering, Scalability and Data SRE. Shiv currently is the Founder, Investor, Board Member and CEO of multiple Database Systems Infrastructure Operations companies in the Transaction Processing Computing and ColumnStores ecosystem. He is also a frequent speaker in open source software conferences globally.