I've run into a problem with a mongodb setup (2.0.2) that is only written to yet I see significant read i/o that slows down the performance quite a bit. The setup is a simple 3-node mongodb cluster (primary, secondary, arbiter) where the primary and secondary nodes have 8gb of ram and run a replicated set.

With a collection size of about 44gb I see a read-to-write ration of 3:1 that is there are actually more reads than writes happening on the system. After renaming the collection and starting a new one from scratch this ration changed to 1:4 which is significantly better but I still don't understand what causes the reads. The entire collection fits in memory right now so I'm not sure what mongo could be reading from the disk all the time.
Once I stop writing to the collection the reads also disappear.