I recently wrote a post on tracking metadata locks (MDL) in MySQL 5.7, and I wanted to take a moment to expand on it by explaining a couple of the associated variables in more detail.

First off, once you have enabled the performance_schema *and* the metadata lock instrumentation, you can verify it with:

mysql> SELECT * FROM performance_schema.setup_instruments
    -> WHERE NAME = 'wait/lock/metadata/sql/mdl';
+----------------------------+---------+-------+
| NAME                       | ENABLED | TIMED |
+----------------------------+---------+-------+
| wait/lock/metadata/sql/mdl | YES     | YES   |
+----------------------------+---------+-------+

“ENABLED” will report “YES” if it is enabled properly, and “NO” if not.

“TIMED” (referring to event timing) reports “YES” if it was enabled via the my.cnf or my.ini file, and it …

I recently blogged about tracking metadata locks in the latest MySQL, and now I want to discuss how to track these metadata locks in MariaDB.

In MySQL 5.7, there is a table named `metadata_locks` added to the performance_schema (performance_schema must be enabled *and* the metadata_locks instrument must be specifically enabled as well.

In the MariaDB 10.0 implementation (as of 10.0.7), there is a table named METADATA_LOCK_INFO added to the *information_schema*. This is a new plugin, so the plugin must be installed, but that is very simple with:

INSTALL SONAME 'metadata_lock_info';

Then, you will have the table.

To see it in action:

Connection #1:

mysql> create table t (id int) engine=myisam;
mysql> begin;
mysql> select * from t;

Connection #2:

mysql> alter table t add …

I’ve blogged about metadata locks (MDL) in the past (1 2 3) and in particular discussed how best to track them down and troubleshoot threads stuck waiting on metadata locks.

If you’ve had any experience with these, you’ll know finding them isn’t always the most straight-forward task.

So I was glad to see metadata lock instrumentation added to MySQL 5.7.3 as part of performance_schema, which makes tracking these down a breeze! (Note this is only in 5.7.3 currently, and therefore is some time from being GA as of today)!

To use these, performance_schema must be enabled (i.e., performance_schema=1 in your config file).

But, also, the metadata_locks instrument is disabled by default, so even if you enable the …

MySQL has exceptional track of record by introducing minor fixes that cause major breakages. Though usually I could blame naiveté of engineers, who did not really ever have to deal with production implications, but lately I can start sensing various business implications against open-source offerings.

As an original author of mydumper I really cannot get out of my mind that 5.5 and 5.6 metadata locking changes are there to screw with anyone who is building a backup solution using stable snapshot views of MySQL (for example, mysqldump –single-transaction, the golden standard of backing things up in MySQL world).

As seen in a bug #71017 (palindrome!) filed by my esteemed colleague Eric, newly introduced behaviors …

If you see the “Waiting for table metadata lock” error, you may be wondering what is the best course of action to prevent it in the future. I have briefly discussed troubleshooting metadata locks before, however, that post more illustrated how easy it is to encounter the “Waiting for table metadata lock” error in both MyISAM and InnoDB. It provides simple examples for reproducing using both storage engines, so one could hopefully more easily identify where the locks are stemming from in their particular case.

Pin-pointing which transaction holds the locks is a different story. There are feature requests filed in MySQL and MariaDB bugs databases to track this information, but they are recent, and no ETA is scheduled for either yet, as far as I know:

http://bugs.mysql.com/bug.php?id=69527 …

While most of the talk recently has mostly been around the new changes in MySQL 5.6 (and that is understandable), I have had lately some very interesting cases to deal with, with respect to the Metadata Locking related changes that were introduced in MySQL 5.5.3. It appears that the implications of Metadata Locking have not been covered well, and since there are still a large number of MySQL 5.0 and 5.1 installations that would upgrade or are in the process of upgrading to MySQL 5.5, I thought it necessary to discuss what these implications exactly are.

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