I've been running Mongodb as a replica set on 3 servers (primary, secondary and arbiter) but because I have to downscale I'm converting the primary to a standalone server. Before this change the replica set secondary was just a "backup" in case of primary failure, so no distribution of reads.
I was expecting the CPU load on my primary server to go down or stay the same. However it went up a bit instead (before my "uptime" load rating would be around 0.01, now it's usually around 0.1- 0.2). Not a problem, but stil significant.
So what exactly is the difference in running mongod as a replica set vs a standalone server? And are there drawbacks to running mongod as a single node "fake" replica set?
I'm using the MongoRocks storage engine.