MySQL 5.6 replication causes ‘waiting for table lock’

The MyISAM Storage Engine is furiously notorious for performing full table locks for any DML (INSERTs, UPDATEs, DELETEs). InnoDB would definitely solve that issue in the long term.

I wrote about pros and cons of using MyISAM vs InnoDB

• InnoDB vs MyISAM with many indexes (Jul 05, 2012)
• Which is faster, InnoDB or MyISAM? (May 03, 2012)
• Any gotchas at all with converting from MyISAM to InnoDB? (Jan 09, 2012)
• Should I use a storage engine other than MyISAM to optimise these tables or should I get better disks? (Oct 07, 2011)
• Benefits of table level locking (Sep 14, 2011)
• What are the main differences between InnoDB and MyISAM? (Apr 14, 2011)

With regard to your current question, here is a possible scenario:

• article and article_comments are both MyISAM tables
• article_comments has one or more indexes with status as a column
• Index page updates for article_comments are cached in the MyISAM Key Buffer (sized by key_buffer_size), causing old index pages out of the MyISAM Key Buffer
• You have SELECT queries that perform JOINs between article and article_comments

In my suggested scenario, SELECTs against the article table can be held up from allowing writes because of having to wait for article_comments to be free from any DML (in this case, an UPDATE)

I don't use pt-table-sync because I don't agree with the approach it uses:

When synchronizing a server that is a replication slave with the –replicate or –sync-to-master methods, it always makes the changes on the replication master, never the replication slave directly. This is in general the only safe way to bring a replica back in sync with its master; changes to the replica are usually the source of the problems in the first place. However, the changes it makes on the master should be no-op changes that set the data to their current values, and actually affect only the replica. Please read the detailed documentation that follows to learn more about this.

^{— http://www.percona.com/doc/percona-toolkit/2.1/pt-table-sync.html}

My philosophy is "no, that's okay, you guys can go ahead and stay away from the master."

While it's true that a common cause of replication errors are related to things being changed directly on the slave, it's also true that if two servers have an identical set of data at an identical set of binlog coordinates, that's a valid place to begin replication. (I would also suggest that the other common cause of replication errors is people trying to replicate only some of their tables and not all, which is almost never a good idea, unless you really know what you're doing, in which case, you'll know better than to try it.)

If you do use it, and I'm sure a lot of people do, then I don't think you actually want to stop the slave. I think you need to keep the slave running to it can make those "no-op changes to the master" and watch them replicate over to fix the inconsistent tables. I don't know what it does with errors, perhaps it skips over them.

Here is one of the alternative approaches that I use.

You need three console windows for this. Do not disconnect your session from the master or you will lose the global lock. You need a console connection to the master and one to the slave, and you need to have your commands to do mysqldump set up and ready to go in a third window.

First, check the master's binlog coordinates and the slave's binlog coordinates to make sure the slave is not lagging.

master-mysql> SHOW MASTER STATUS;

+-----------------------+-----------+--------------+------------------+
| File                  | Position  | Binlog_Do_DB | Binlog_Ignore_DB |
+-----------------------+-----------+--------------+------------------+
| pri-master-bin.004421 | 142052212 |              |                  |
+-----------------------+-----------+--------------+------------------+

slave-mysql> SHOW SLAVE STATUS\G

    Relay_Master_Log_File: pri-master-bin.004421
      Exec_Master_Log_Pos: 142052212
         Slave_IO_Running: Yes
        Slave_SQL_Running: Yes
    Seconds_Behind_Master: 0

There are more values included with SHOW SLAVE STATUS but these are the important ones. The Exec_Master_Log_Pos needs to be close to or identical to the master, and the log file the same.

Next, obtain a brief read lock on the master. This locks all of the tables for write allows the master to settle down long enough that we know the slave and master are at the same place in the binlog at this point in time.

master-mysql> flush tables with read lock;
Query OK, 0 rows affected (24.26 sec)

master-mysql>

Note that this sometimes takes a few seconds before your prompt returns.

Do not disconnect this session. You need this session active because it is holding the lock on the master. You can use it for the following, but don't close it.

Now, repeat the SHOW MASTER STATUS; on the master and SHOW SLAVE STATUS\G on the slave. You should find that they have settled down to the same set of binlog coordinates. Wait until they do. If they don't settle down quickly, you might want to drop the read lock on the master to figure out why not.

Next:

slave-mysql> STOP SLAVE SQL_THREAD;

This will stop the slave from executing any more events from the master, which is still locked.

In the third window, start a backup of the tables from the master.

shell> mysqldump --verbose --single-transaction database_name table1 table2 table3 table4 > dump.sql

-- Connecting to *hostname*...
-- Starting transaction...
-- Retrieving table structure for table...

As soon as you see "Retrieving table structure..." you can release the global read lock on your master, so your application can start writing to it again.

mysql-master> unlock tables;
Query OK, 0 rows affected (0.01 sec)

If your tables are InnoDB, the --single-transaction option should get a consistent backup from the master at a point in time during the few seconds that you had the global read lock on the master. Since you also stopped the slave at that point, the slave should be positioned at precisely the appropriate point in time for applying that backup.

When the backup of those tables is complete, apply it to the slave:

shell> mysql --host=slave.host.name database_name < dump.sql

At this point, you have restored the tables onto the slave in a form that should be identical to the way they appeared on the master at the time when the slave was stopped, and replication should resume when you remove the configuration not to replicate those tables, and then:

slave-mysql> START SLAVE SQL_THREAD;

Be sure your backups are solid before you begin.