MySQL 5.5 lock_wait_timeout: patience is a virtue, and a locked server

Like Ovais said in Implications of Metadata Locking Changes in MySQL 5.5, the hot topic these days is MySQL 5.6, but there was an important metadata locking change in MySQL 5.5.  As I began to dig into the Percona Toolkit bug he reported concerning this change apropos pt-online-schema-change, I discovered something about lock_wait_timeout …

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 PROCESSLIST was captured at the time.

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