MongoDB session database performance considerations
How to calculate your session database hardware requirements.
For each web request, the content delivery server accesses the session-state store database multiple times. This can have a significant impact on the performance of your website. Therefore, you should use solid-state drives for the database and install enough memory to avoid frequent disk reads.
If you are using a MongoDB database as your session-state store, you should think about the following hardware and performance considerations.
Each visit requires about 30 kilobytes of storage capacity in the session database (by default) but space requirements may change depending on customizations.
You can calculate the disk space requirements expressed in kilobytes using the following formula:
Maximum number of concurrent visits * 30
This includes active visits and inactive visits that have not timed out.
You can also use the same formula to calculate memory requirements.
The MongoDB website has more information on how to calculate your RAM requirements and also on installation steps: