Difference between mongodb and tar storage?
why we use mongodb?
how to setup mongodb storage?
advantage of mongodb?
steps to reduce data by revision clean up to maintain the repository?
You can get enough information in the below Adobe Docs:
From AEM 6.0, We are using repository as OAK not TAR. Also, additionally you can use Mango-db as a additional repository
More information in this thread: Which option is best with AEM 6.2 between the TarMK and MongoDB
Also there is AEM GEMS Session on AEM with MangoDB: Running AEM on MongoDB
Difference between TAR and MANGODB: Adobe Experience Manager - CQ Tutorials: AEM Microkernels - MongoMK vs TarMK
Hope this helps!!