February 17, 2010

Setting up Munin on Ubuntu

Posted in Software at 06:50 by graham

Munin is a system monitoring tool. It produces graphs for Apache, MySQL, Nginx, CPU, Memory, Disk, etc. Example munin installation – Live.

Here are my notes from setting it up, they are brief, but should help you get going.

All the monitored machines run a small daemon called munin-node. One machine is the central server. Every few minutes it gathers data from all the nodes (including itself), generates the graphs, and writes out some HTML files.

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February 14, 2010

Restarting MySQL master-master replication

Posted in Software at 21:24 by graham

If your MySQL (5.0+) replication is broken, there’s two ways to fix it: The easy way, and the right way.

Run commands starting with $ on Unix. Run commands starting with mysql> in the MySQL client.

The easy way: Skip the problem

If you hit both databases at the same time, with the same INSERT, they will create their own record, and try and replicate to the other, which already has that record, causing a duplicate error.

In a simple case like that, you just want to skip the offending statement:

mysql>SET GLOBAL SQL_SLAVE_SKIP_COUNTER=1; START SLAVE;

More details on skipping MySQL duplicate errors

Most of the time, you skip one statement, and replication breaks again straight away, because there’s a whole queue of problem statements coming up.

The right way: Rebuild

If you are not sure that you can skip the duplicate, or if replication has been broken long enough that your two servers are out of synch, pick one database to be the master, and rebuild the other from a copy of that master.

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