Hi! I have mongodb version 2.6.10 on ubuntu 16.04. And want to upgrade to 4.0. But I don’t want to upgrade from 2.6 to 3.0, from 3.0 to 3.2, etc as mongodb documentation said. I didn’t found what graylog stores in mongodb. May I just remove the old version and install the latest 4.0?
@Nessero Graylog uses MongoDB to store your configuration data, not your log data. Only metadata is stored, such as user information or stream configurations. None of your log messages are ever stored in MongoDB. This is why MongoDB does not have a big system impact, and you won’t have to worry too much about scaling it. With our recommended setup architecture, MongoDB will simply run alongside your Graylog server processes and use almost no resources.
If you have any questions about Graylog, MongoDB and Elasticsearch then take a look at below documentation links.
https://docs.graylog.org/en/latest/pages/faq.html#frequently-asked-questions
Hope this helps you for your MongoDB upgrade related query 
I would suggest to backup data in 2.6.10, install 4.0 and then restore data
Big thank for helping me ! So I will lose all my graylog accounts if I just delete the current version ?
Thanks for your reply, i will try this solution too.
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