Feature Overview
During execution of a DML statement, if a metadata lock (MDL) is held, it will block DDL operations for the corresponding table and then block all subsequent DML statements, possibly consuming a large amount of thread resources. To prevent this case, TDSQL-C for MySQL introduces a Nonblocking DDL mechanism, protecting the system from being stalled by DDL operations waiting for MDL locks. When a DDL operation is blocked by a DML statement, subsequent DML statements will not further get blocked due to waiting for an MDL lock.
Supported Versions
The kernel version should be 3.1.15 or later for TXSQL 8.0.
Applicable Scenarios
It is applicable to scenarios where an MDL lock held during execution of a DML statement may block DDL operations for the corresponding table and then block all subsequent DML statements.
Use Instructions
|
txsql_nonblock_ddl | yes | bool | false | true/false | Whether to enable the Nonblocking DDL feature. |
txsql_nonblock_ddl_retry_times | yes | ulong | 1 | 0-ulong_max | Retry count after a DDL operation is blocked by an MDL lock. The default value is 1. |
txsql_nonblock_ddl_retry_interval | yes | ulong | 2 | 0-ulong_max | Retry interval after a DDL operation is blocked by an MDL lock. The default value is 2s. |
Note:
Some uncommon DDL operations may not be covered. In these scenarios, if a DDL operation is blocked by a DML statement, the system will directly return Lock wait timeout.