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 …

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 reports …

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 index …

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 …