Running many mongod instances on one server

[Alexandre M]

Hi,
I'd like to have your thoughts about a possible deployment of MongoDB.

Some people at my company want us to deploy many mongod daemons on the
same single server (on EC2) in order to better use multiple cores.

I think this is a bad idea.

First, I don't think it's gonna save us a lot of money, but more
importantly I think it's just not going to be efficient performance
wise.

Considering the memory mapped file usage of MongoDB, I think this is
going to cause trouble. It seems like having many instances running
would mean constant fighting between those mongod and the kernel.
Basically, it seems like one mongod can occupy all the available
memory on the system, there's no way to cap its usage.

Am I right to believe this? Have any of you deployed Mongo in
production this way?

[Kristina Chodorow]

You are correct, this is generally a bad idea because the mongod
instances will compete for memory (unless, of course, you have enough
memory for them all). It depends on how fast you need things to go,
though: if fairly regular disk access (slowness) is okay, then it
should work fine. (One mongod wouldn't dominate unless it was the
only one reading/writing: everyone requesting data would get some
memory.)