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From:Baron Schwartz Date:October 8 2007 3:55pm
Subject:How to get sane error log behavior
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I had a server crash over the weekend and discovered that I don't know 
as much about MySQL error logging as I thought.  The error logs were 
empty because of FLUSH LOGS.

What I learned today is if I specify the log-errors parameter in 
/etc/mysql/my.cnf, every time there's a FLUSH LOGS (or mysqld catches a 
SIGHUP), it rotates the error log to -old, like this:

Here's the config file line:
log-error = /var/log/mysql/mysqld.err

And here's the directory:

$ ls -l /var/log/mysql/
-rw-rw---- 1 mysql mysql       0 Jul 11 08:40 mysql.err
-rw-rw---- 1 mysql mysql 1617643 Oct  8 11:26 mysqld.err
-rw-rw---- 1 mysql mysql   61984 Oct  8 11:22 mysqld.err-old

I created those logs by deliberately making InnoDB spit out some output 
with the InnoDB Lock Monitor, then did a FLUSH LOGS, and mysqld itself 
moved mysqld.err to mysqld.err-old.

If I don't specify log-error, it doesn't log anything (duh).  In this 
case, the output goes where?  to mysql.err instead of mysqld.err?  No, 
that file is created by mysqld_safe and never seems to get anything 
written to it.  It seems to go to /dev/null, or maybe it just isn't 
logging anything at all when log-error isn't specified.

The problem here is that the renaming is throwing away my error 
messages, which I need to debug a problem.  And I don't just want this 
file written in the data directory, so I don't want to omit the filename 
and let MySQL choose where the file goes (which appears to be the only 
way to avoid the auto-renaming behavior, according to the manual section 
5.11.1).

I do not like this auto-rotating behavior.  I want to use logrotate to 
handle this.  Is there any way I can configure MySQL to just log to the 
specified file, do a close-and-reopen when I run FLUSH LOGS like it does 
for the other logs, and use logrotate to rotate the files myself?

Thanks
Baron
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How to get sane error log behaviorBaron Schwartz8 Oct