multiple instances of Mongo DB on same server

Question

The aforementioned answer is not a recommended way to run multiple instances (especially when the servers might be running at the same time) as it will lead to usage of the same config parameters like for example logpath and pidfilepath which in most cases is not what you want.

Please, consider creating dedicated mongod configuration files like mongod-QA.conf and mongod-STAGE.conf. In these files you may want to provide dbpath, logpath folders, bind_ip, port and pidfilepath specific to each mongod instance and that will not affect each other.

After these steps you are good to trigger two instances as follows

mongod --config <path-to>/mongod-QA.conf
mongod --config <path-to>/mongod-STAGE.conf

You can find more details on mongodb docs page

You just need to create another folder(ex: mongodb2) dbpath for the second instance, and run it in a different port(ex: 27018)

 mongod --dbpath /usr/local/var/mongodb2 --port 27018

Here is how I start 4 mongod's on the same pc to emulate production environment in development environment.

To start mongod you should use separate config for each mongod. Take 4 configs and start mongods using them:

start C:\mongodb\bin\mongod.exe --config C:\net2\dev1-pc\configs\mongod-primary1.cfg 
start C:\mongodb\bin\mongod.exe --config C:\net2\dev1-pc\configs\mongod-secondary1.cfg --rest
start C:\mongodb\bin\mongod.exe --config C:\net2\dev1-pc\configs\mongod-secondary2.cfg
start C:\mongodb\bin\mongod.exe --config C:\net2\dev1-pc\configs\mongod-secondary3.cfg

Configs look like this:

mongod-primary1.cfg file contents

systemLog:
    destination: file
    path: c:\net2\primary1-pc\data\log\mongod.log
storage:
    dbPath: c:\net2\primary1-pc\data\db
net:
    port: 27018
replication:
    replSetName: repl1

mongod-secondary1.cfg file contents

systemLog:
    destination: file
    path: c:\net2\secondary1-pc\data\log\mongod.log
storage:
    dbPath: c:\net2\secondary1-pc\data\db
net:
    port: 27019
replication:
    replSetName: repl1

mongod-secondary2.cfg file contents

systemLog:
    destination: file
    path: c:\net2\secondary2-pc\data\log\mongod.log
storage:
    dbPath: c:\net2\secondary2-pc\data\db
net:
    port: 27020
replication:
    replSetName: repl1

mongod-secondary3.cfg file contents

systemLog:
    destination: file
    path: c:\net2\secondary3-pc\data\log\mongod.log
storage:
    dbPath: c:\net2\secondary3-pc\data\db
net:
    port: 27021
replication:
    replSetName: repl1

It's possible - you would give each one its own port to listen on, and its own --dbpath directory to put its files in, but I wouldn't recommend this because they will both be competing for the same resources - RAM, i/o bandwidth, etc.

If you have multiple disks on this server you can place their data files on separate devices but you're still risking your QA instance reducing availability of the production instances, possibly at the worst possible time.

I would put QA instance on a random machine that's doing something unimportant before I would colocate it with my production instance.