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RDS for MySQL

Feature

Non-blocking DDL

This topic describes the non-blocking DDL feature of RDS for MySQL.

Supported version

MySQL 8.0, and the kernel minor version of MySQL 8.0.43_20260815 and above.

tip

You can view the kernel minor version of an instance by Viewing instance information. If the kernel minor version of your instance is not within the supported kernel minor version range, you can Manually upgrading the instance kernel minor version.

Background

In MySQL, DDL operations must acquire the MDL lock on the target table to commit metadata changes and ensure metadata consistency. MDL locks are incompatible with locks of any other type. If the DDL thread enters the pending state because it cannot acquire the lock immediately, all subsequent accesses to the target table will be blocked, which may cause business system failures and unavailability.

Feature description

After non-blocking DDL is enabled, for DDL threads that need to upgrade to an MDL lock, the system attempts to acquire the lock through intermittent retries while waiting for the MDL lock and does not block continuously. Between two attempts to acquire the lock, business DDL operations can still continue to execute.

Limitation

Enabling the non-blocking DDL feature reduces the priority of DDL. Therefore, the probability of DDL failures caused by the inability to acquire the MDL lock increases.

Only ALTER TABLE, CREATE INDEX and DROP INDEX operations are currently supported.

RENAME TABLE cannot be executed directly.

It does not take effect in the slave_thread scenario.

Instructions

You can enable and adjust the non-blocking DDL feature through the loose_rds_nonblock_ddl_retry_interval and loose_rds_nonblock_ddl_lock_wait_timeout parameters.

Parameter name	Default value	Whether an instance restart is required for the change to take effect	Value range	Scope	Parameter description
loose_rds_nonblock_ddl_retry_interval	0	No	0~31536000	GLOBAL / SESSION	The time interval for the DDL thread to retry when acquiring the MDL lock times out. The default value is 0, which means the non-blocking DDL feature does not take effect. The unit is seconds.
loose_rds_nonblock_ddl_lock_wait_timeout	1	No	1~31536000	GLOBAL / SESSION	The timeout period for the DDL thread to intermittently acquire the MDL lock. The default value is 1, and the unit is second.

Performance test

Test Environment

Instance specification: RDS for MySQL 4C8G.

Instance version: MySQL 8.0.

Test steps

Create tables and insert data through sysbench.

Start the sysbench stress test to simulate online business.

Start session 1 and open a long-running transaction on the target table, to block subsequent DDL operations on this table.

Start another session 2 and execute the DDL operation with the non-blocking DDL feature disabled.

Enable the non-blocking DDL feature, and set the loose_rds_nonblock_ddl_retry_interval parameter to 6 and the loose_rds_nonblock_ddl_lock_wait_timeout parameter to 1. Execute the same DDL operation as in the previous step.

The test results are as follows.

The following conclusions can be drawn from the comparison figure below:

When the non-blocking DDL feature is disabled, the DDL thread cannot acquire the MDL lock, and the session is fully blocked.

When the non-blocking DDL feature is enabled, the DDL thread can acquire the MDL lock intermittently, and the session is not fully blocked.

Last updated: 2026.10.16 16:50:18