I enabled the InnoDB Lock Monitor in my MariaDB 5.5 instance (using XtraDB+ as the InnoDB – which is the default in MariaDB) and noticed that while the SHOW ENGINE INNODB STATUS was being logged to the error log, it wasn’t logging the “additional” lock information – it just looked like the plain ‘ole INNODB STATUS.

Long story short, Percona added a new variable so one has better control over what gets logged:

innodb_show_verbose_locks

If off (default), then the InnoDB Lock Monitor logs the normal INNODB STATUS, and if enabled, then it logs it with the extended lock information.

They also created another variable that goes along with this one (and the InnoDB Lock Monitor), which is:

innodb_show_locks_held

This variable indicates the number of locks to print that are held …

One common cause for deadlocks when using InnoDB tables is from the existence of foreign key constraints and the shared locks (S-lock) they acquire on referenced rows.

The reason I want to discuss them though is because they are often a bit tricky to diagnose, especially if you are only looking at the SHOW ENGINE INNODB STATUS output (which might be a bit counter-intuitive since one would expect it to contain this info).

Let me show a deadlock error to illustrate (below is from SHOW ENGINE INNODB STATUS\g):

------------------------
LATEST DETECTED DEADLOCK
------------------------
111109 20:10:03
*** (1) TRANSACTION:
TRANSACTION 65839, ACTIVE 19 sec, OS thread id 4264 starting index read
mysql tables in use 1, locked 1
LOCK WAIT 6 lock struct(s), heap size 1024, 3 row lock(s), undo log entries 1
MySQL thread id 3, query id 74 localhost 127.0.0.1 root Updating
UPDATE parent SET age=age+1 WHERE id=1
*** (1) WAITING FOR THIS LOCK TO BE …

Tracking down InnoDB lock information using the new Information Schema tables provided with the InnoDB plugin has never been easier.

Long story short, the other day I was trying to identify what transaction was holding the lock for a particular UPDATE. The UPDATE would not complete and kept timing out with “Lock wait timeout exceeded; try restarting transaction”.

Of course I checked the output of SHOW ENGINE INNODB STATUS. From that output, I could tell it was the replication thread holding the lock, *since* it was the only other transaction running. But I could not verify this with output, it just had to be the case.

Next I enabled the InnoDB lock monitor, and examined that output too. Here, it showed more detail on the locks being held, however, it only shows the first 10 locks held by the replication thread. In this case, the …