MySQL Replication Failed

tom — Sun, 26 Apr 2009 12:40:28 +0000

I wanted to replicate a MySQL-Database using the integrated Master -> Slave mechanism.

Master-System: openSUSE 10.2, MySQL 5.0.26
The Slave-Setup: openSUSE 11.1, MySQL 5.0.67

This is quite easy to setup usually. This time, I almost had a nervous breakdown.
On the slave side the replication just didn’t start. The logfile showed these errors:
090421 20:49:28 [ERROR] Slave: Error 'Duplicate entry '790233' for key 1' on query. ... 090421 20:49:28 [ERROR] Error running query, slave SQL thread aborted. Fix the problem, and restart the slave SQL thread with "SLAVE START". We stopped at log 'mysql-bin.000008' position 30408893

As checking the database and making sure that there were no “duplicate entries”,
did not bring up any results, I found something curious (after hours and hours).

On the Master-System:

mysql> SHOW MASTER STATUS; +- - - - - - - - + - - - - - + - - - - - - - + - - - - - - - - -+ | File | Position | Binlog_Do_DB | Binlog_Ignore_DB | +- - - - - - - - + - - - - - + - - - - - - - + - - - - - - - - -+ | mysql-bin.0001 | 10034 | mydb,mydb | | +- - - - - - - - + - - - - - + - - - - - - - + - - - - - - - - -+
Why is the database listed two times? In the configuration it’s only used one time:

binlog-do-db = mydb

After spending some time on google, I found the reason:
http://bugs.mysql.com/bug.php?id=20748
It’s a bug : / MySQL (in that version) is reading the configuration file twice.

If installing a newer version is not possible, you can use this workaround:
Start MySQL with the option --defaults-file=/etc/my.cnf
This advices MySQL to ONLY use this config file.

Increase MySQL Performance

tom — Mon, 14 May 2007 18:11:29 +0000

Keyword: MySQL Query Cache

If you want to increase the speed of your MySQL Database (answers), think about activating the Query Cache. That is available since Version 4. It’s really recommendable when you are working a lot with dynamic generated content, that does not change too often. Usually that is the fact for most webservers. In that case the Query Cache will give you back the answers for your sql queries without polling the database – as long as the tables do not change.

Insert in your config (/etc/my.cnf) i.e.:
query_cache_size = 8M

You can control the usage with a sql query:
SHOW STATUS LIKE 'Qcache%';

Thanks to Thomas Schefter for the hint.

tom's blog » Databases

MySQL Replication Failed

Increase MySQL Performance