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Displaying posts with tag: interactive_timeout (reset)

Mitigating the Effect of Metadata Lock (MDL) Contention
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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:

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Using SHOW PROCESSLIST and mysqladmin debug Output in Conjunction with SHOW INNODB STATUS
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When InnoDB appears hung, I know the natural reaction is to check SHOW ENGINE INNODB STATUS.

In fact, it’s the first thing I check when InnoDB tables are involved.

However, I just want to iterate how valuable SHOW FULL PROCESSLIST and/or mysqladmin debug outputs can be even when it seems mysqld is hung on on InnoDB table.

Two recent cases I’ve encountered illustrate why.

Case #1:

MySQL appeared hung on the following simple, single-row INSERT:

---TRANSACTION 0 2035648699, ACTIVE 76629 sec, process no 9047,
OS thread id 3069426592, thread declared inside InnoDB 500
mysql tables in use 1, locked 1
...
INSERT INTO test (id, parent, text) VALUES (180370, 70122, 'test table')

At least that’s what it seemed per the INNODB STATUS, but unfortunately, there wasn’t any further information to go on.

The next time it occurred, SHOW FULL

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Fatal timeout !
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There are several parameters to set a timeout on MySQL :

But I would like to focus on wait_timeout (or interactive_timeout depending on how you connect)

This timeout allows MySQL to automatically close a connection in case of non-activity during the time defined by this parameter (default value is 28000 seconds).

The problem I wish to explain here may happen when this timeout is set to a low value (about 10 to 30 seconds).

Indeed, in this case, this timeout may have serious consequences, look at that :
[The wait_timeout parameter is set to 10 seconds in this case]

You are connected through the standard MySQL client and you


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Showing entries 1 to 3

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