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Quickly Troubleshooting Metadata Locks in MySQL 5.7

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Quickly Troubleshooting Metadata Locks in MySQL 5.7

If your metadata locks are giving you problems, there are several ways to check, kill, and pass the work onto other threads to speed things along.

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In a previous article, Ovais demonstrated how a DDL can render a table blocked from new queries. In another article, Valerii introduced performance_schema.metadata_locks, which is available in MySQL 5.7 and exposes metadata lock details. Given this information, here’s a quick way to troubleshoot metadata locks by creating a stored procedure that can:

  • Find out which thread(s) have the metadata lock.
  • Determine which thread has been waiting for it the longest.
  • Find other threads waiting for the metadata lock.

Setting Up Instrumentation

First, you need to enable instrumentation for metadata locks:

UPDATE performance_schema.setup_instruments SET ENABLED = 'YES' WHERE NAME =

'wait/lock/metadata/sql/mdl';

Second, you need to add this stored procedure:

USE test;
DROP PROCEDURE IF EXISTS procShowMetadataLockSummary;
delimiter //
CREATE PROCEDURE procShowMetadataLockSummary()
BEGIN
	DECLARE table_schema VARCHAR(64);
    DECLARE table_name VARCHAR(64);
    DECLARE id bigint;
    DECLARE time bigint;
    DECLARE info longtext;
	DECLARE curMdlCount INT DEFAULT 0;
    DECLARE curMdlCtr INT DEFAULT 0;
	DECLARE curMdl CURSOR FOR SELECT * FROM tmp_blocked_metadata;
	DROP TEMPORARY TABLE IF EXISTS tmp_blocked_metadata;
	CREATE TEMPORARY TABLE IF NOT EXISTS tmp_blocked_metadata (
       table_schema varchar(64),
       table_name varchar(64),
       id bigint,
	   time bigint,
       info longtext,
       PRIMARY KEY(table_schema, table_name)
    );
    REPLACE tmp_blocked_metadata(table_schema,table_name,id,time,info) SELECT mdl.OBJECT_SCHEMA, mdl.OBJECT_NAME, t.PROCESSLIST_ID, t.PROCESSLIST_TIME, t.PROCESSLIST_INFO FROM performance_schema.metadata_locks mdl JOIN performance_schema.threads t ON mdl.OWNER_THREAD_ID = t.THREAD_ID WHERE mdl.LOCK_STATUS='PENDING' and mdl.LOCK_TYPE='EXCLUSIVE' ORDER BY mdl.OBJECT_SCHEMA,mdl.OBJECT_NAME,t.PROCESSLIST_TIME ASC;
    OPEN curMdl;
    SET curMdlCount = (SELECT FOUND_ROWS());
    WHILE (curMdlCtr < curMdlCount)
    DO
      FETCH curMdl INTO table_schema, table_name, id, time, info;
      SELECT CONCAT_WS(' ','PID',t.PROCESSLIST_ID,'has metadata lock on', CONCAT(mdl.OBJECT_SCHEMA,'.',mdl.OBJECT_NAME), 'with current state', CONCAT_WS('','[',t.PROCESSLIST_STATE,']'), 'for', t.PROCESSLIST_TIME, 'seconds and is currently running', CONCAT_WS('',"[",t.PROCESSLIST_INFO,"]")) AS 'Process(es) that have the metadata lock' FROM performance_schema.metadata_locks mdl JOIN performance_schema.threads t ON t.THREAD_ID = mdl.OWNER_THREAD_ID WHERE mdl.LOCK_STATUS='GRANTED' AND mdl.OBJECT_SCHEMA = table_schema and mdl.OBJECT_NAME = table_name AND mdl.OWNER_THREAD_ID NOT IN(SELECT mdl2.OWNER_THREAD_ID FROM performance_schema.metadata_locks mdl2 WHERE mdl2.LOCK_STATUS='PENDING' AND mdl.OBJECT_SCHEMA = mdl2.OBJECT_SCHEMA and mdl.OBJECT_NAME = mdl2.OBJECT_NAME);
      SELECT CONCAT_WS(' ','PID', id, 'has been waiting for metadata lock on',CONCAT(table_schema,'.', table_name),'for', time, 'seconds to execute', CONCAT_WS('','[',info,']')) AS 'Oldest process waiting for metadata lock';
      SET curMdlCtr = curMdlCtr + 1;
	  SELECT CONCAT_WS(' ','PID', t.PROCESSLIST_ID, 'has been waiting for metadata lock on',CONCAT(table_schema,'.', table_name),'for', t.PROCESSLIST_TIME, 'seconds to execute', CONCAT_WS('','[',t.PROCESSLIST_INFO,']')) AS 'Other queries waiting for metadata lock' FROM performance_schema.metadata_locks mdl JOIN performance_schema.threads t ON t.THREAD_ID = mdl.OWNER_THREAD_ID WHERE mdl.LOCK_STATUS='PENDING' AND mdl.OBJECT_SCHEMA = table_schema and mdl.OBJECT_NAME = table_name AND mdl.OWNER_THREAD_ID AND t.PROCESSLIST_ID <> id ;
	END WHILE;
    CLOSE curMdl;
END//
delimiter ;


Testing

Now, let’s call the procedure to see if there are threads waiting for metadata locks:

mysql> CALL test.procShowMetadataLockSummary();
+----------------------------------------------------------------------------------------------------------------+
| Process(es) that have the metadata lock                                                                        |
+----------------------------------------------------------------------------------------------------------------+
| PID 10 has metadata lock on sbtest.sbtest with current state [] since 274 seconds and is currently running []  |
| PID 403 has metadata lock on sbtest.sbtest with current state [] since 291 seconds and is currently running [] |
+----------------------------------------------------------------------------------------------------------------+
2 rows in set (0.00 sec)
+------------------------------------------------------------------------------------------------------------------------+
| Oldest process waiting for metadata lock                                                                               |
+------------------------------------------------------------------------------------------------------------------------+
| PID 1264 has been waiting for metadata lock on sbtest.sbtest for 264 seconds to execute [truncate table sbtest.sbtest] |
+------------------------------------------------------------------------------------------------------------------------+
1 row in set (0.00 sec)
+---------------------------------------------------------------------------------------------------------------------------+
| Other queries waiting for metadata lock                                                                                   |
+---------------------------------------------------------------------------------------------------------------------------+
| PID 1269 has been waiting for metadata lock on sbtest.sbtest for 264 seconds to execute [SELECT c from sbtest where id=?] |
| PID 1270 has been waiting for metadata lock on sbtest.sbtest for 264 seconds to execute [SELECT c from sbtest where id=?] |
| PID 1271 has been waiting for metadata lock on sbtest.sbtest for 264 seconds to execute [SELECT c from sbtest where id=?] |
| PID 1272 has been waiting for metadata lock on sbtest.sbtest for 264 seconds to execute [SELECT c from sbtest where id=?] |
| PID 1273 has been waiting for metadata lock on sbtest.sbtest for 264 seconds to execute [SELECT c from sbtest where id=?] |
+---------------------------------------------------------------------------------------------------------------------------+
5 rows in set (0.00 sec)


So, as you can see above, you have several choices. You could (a) do nothing and wait for threads 10 and 403 to complete, and then thread 1264 can get the lock.

If you can’t wait, you can (b) kill the threads that have the metadata lock so that the TRUNCATE TABLE in thread 1264 can get the lock. Although, before you decide to kill threads 10 and 403, you should check SHOW ENGINE INNODBSTATUS to see if the undo log entries for those threads are high. If they are, rolling back these transactions might take a long time.

Lastly, you can instead (c) kill the DDL thread 1264 to free up other queries. You should then reschedule the DDL to run during off-peak hours.

Happy metadata lock hunting!

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Topics:
database ,mysql ,database performance ,metadata locks

Published at DZone with permission of Jaime Sicam, DZone MVB. See the original article here.

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