Why did upgrading Drupal 6 cause a huge increase in database traffic? Last week I inherited a several-year old messy implementation of a mid-sized Drupal site. Short version: 1 NFS/mysql server, 3 Apache servers. Everything was overloaded and crawling along. The first thing I did was add a memcached server to the mix which cut the network traffic to the mysql server a lot. Next I got the code all running on local file systems using VCS and only used the NFS mounts for the files directory. Again, network traffic went down. 

The CMS was actually an extended install of Pressflow dated at v6.20. I had the bright idea to update this to 6.26, which I did using the Pressflow 6.26 package. Other than a few conflicts with other modules that had to be resolved, this went pretty smoothly. The site works fine.

However, looking at the interface stats on the mysql server, since the upgrade the ethernet traffic is about 100% more to handle the same site load. It's been three days now and the pattern is the same. The network traffic hugely increased corresponding to the upgrade to 6.26. The metric I'm using here is bytes transferred in and out of the DB server. Unfortunately I don't have stats about number of queries run.

What changed between 6.20 and 6.26 that would cause this and is there anything I can do to mitigate it?
Apart from the NewRelic suggestion, which I put in a comment, consider reproducing this issue in a second environment, I assume you do have a staging/testing environment of the site, or at least a local installation. If not, you should get one ;)

Then you can install the devel.module, enable query logging and/or xhprof profiling. Then you can actually switch between the old version and the new one and compare the difference within Drupal. Devel's query logging will tell you exactly which queries are executed and how long they took.