Do you face some performance issues in your MongoDB setup?
In this case use the following steps to provide some first aid to your system and gain some space for a long term architecture (such as Sharding)
Step 1: Enable slow queries
Get intelligence about your system behavior and performance bottlenecks. Usually there is a high correlation between the slow queries and your performance bottleneck, so use the following method to enable your system profiling collection:
Step 2: Use explain
Explore the problematic queries using explain. You can also use mtools to analyze the logged queries to find high frequent ones.
Step 3: Create indexes
Your analysis should result with new indexes in order to improve the queries
Don't forget to use index buildup in the background to avoid collections locking and system downtime.
Step 4: Use sparse indexes to reduce the size of the indexes
If you use sparse documents, and heavily using the $exists key words in your queries, using sparse indexes (that includes only documents that includes your field) can minimize your index size the boost your query performance.
Step 5: Use secondary preferred to offload queries to slaves
You probably have a replica set and it's waste of resources not using your slaves for read queries (especially for reporting and search operations).
By changing your connection string to secondary preferred, your application will try to run read queries on the slaves before doing that on your master.
Using these simple method, you can gain time and space before hitting the wall.