MySQL 8.0 Performance Regression: Default Parameter Change Causes 3x Slower Table Rebuilds

Background

MySQL 8.0 users have reported significant performance regressions in table rebuild operations (ALTER TABLE ... FORCE) since version 8.0.20. Benchmarks reveal these operations run ​3–5 times slower​ compared to MySQL 5.7, even on identical hardware. The root cause was identified as a flawed default configuration for innodb_doublewrite_pages, introduced in MySQL 8.0.20.

Problem Analysis

The innodb_doublewrite_pages parameter controls how many pages InnoDB flushes to the doublewrite buffer during I/O operations. In MySQL 5.7, this value defaulted to ​128, optimizing throughput by batching writes. However, MySQL 8.0.20+ reset the default to ​4, leading to:

• ​Frequent I/O operations: Smaller batches increase disk latency.
• ​Reduced buffer efficiency: Doublewrite buffer flushes become bottlenecked.
• ​Higher sync costs: Disk synchronization overhead spikes under load.

This change disproportionately impacts operations involving large sequential writes, such as:

• ALTER TABLE ... FORCE (in-place rebuilds).
• DDL operations on tables with millions of rows.
• High-concurrency write workloads.

Historical Context

Prior to MySQL 8.0.20, innodb_doublewrite_pages lacked an explicit default, relying on internal heuristics tied to the buffer pool size. For most deployments, this effectively defaulted to ​128. MySQL 8.0.20+ changed this logic, breaking backward compatibility without adjusting the default.

Affected Scenarios

The regression is most pronounced in:

1. ​Table rebuilds: ALTER TABLE ... ENGINE=InnoDB or ALTER TABLE ... FORCE.
2. ​Large-scale DDL: Adding indexes to wide tables.
3. ​High-write environments: OLTP systems with sustained insert/update rates.

Workarounds and Fixes

Immediate Mitigation (MySQL 8.0.x):

Add the following to your MySQL configuration and restart the instance:

innodb_doublewrite_pages = 128

‍

Monitor performance metrics like Innodb_dblwr_pages_written and Innodb_dblwr_writes to validate improvements.

Long-Term Solution:

Upgrade to ​MySQL 8.4.0 or newer, where the default innodb_doublewrite_pages is restored to ​128.

Key Takeaways

• ​Avoid blind upgrades: Test thoroughly when moving to MySQL 8.0 LTS releases.
• ​Benchmark custom workloads: Parameter defaults may not align with your use case.
• ​Leverage monitoring: Track InnoDB doublewrite activity to preempt regressions.