Recently I faced some issues with Mongoid when upgrading my Rails app from REE+Passenger to MRI 1.9.3+Unicorn.

There are some Resque workers in the background. After some deploy they started to consume a ton of traffic from MongoDB. After some investigation, I found that they heavily read system.namespaces collection. I tried upgrading to latest versions of mongoid(2.4.3) and mongo(1.5.2) to no avail. This does not happen with normal unicorn workers. This also does not happen if I downgrade mongoid to 2.0.1.

I am still not sure what’s happening here. I’ll update this post when I discover something.