What is mongodb behavior regarding keeping loaded indexes in ram?

Question

Say I have a single collection in mongodb with only one index, and I require the index for the entire life cycle of the application using that mongo collection. I would like to know about the behaviour of mongodb. In this case once the index is loaded into memory, will mongodb keep it in the ram?

Thanks

Answer 1

The first thing MongoDB will knock out of RAM will be the LRU ( least recently used ) piece of data. So if you only have one index, chances are it will continue to be used pretty regularly and it should stay in memory.

Source

Answer 2

Unfortunately you cannot currently pin a collection or index in memory. MongoDB uses memory mapped files to load collections and indexes into memory. As your activities touch various pieces of your database thru queries, updates, insertions and deletions, that data will get loaded into memory. This is referred to as the working set. If the total memory required to load the working set is less than available memory, no problem.

If not, MongoDB is going to use an LRU algorithm to pick what to unload from memory. This is why it's so important to understand the concept of the working set and how it relates to your available memory.

This writeup from the documentation should be helpful:

How do I calculate how much RAM I need for my application?

The amount of RAM you need depends on several factors, including but not limited to:

• The relationship between database storage and working set.
• The operating system's cache strategy for LRU (Least Recently Used)
• The impact of journaling
• The number or rate of page faults and other MMS gauges to detect when you need more RAM

Each database connection thread will need up to 1 MB of RAM. MongoDB defers to the operating system when loading data into memory from disk. It simply memory maps all its data files and relies on the operating system to cache data. The OS typically evicts the least-recently-used data from RAM when it runs low on memory. For example if clients access indexes more frequently than documents, then indexes will more likely stay in RAM, but it depends on your particular usage.

To calculate how much RAM you need, you must calculate your working set size, or the portion of your data that clients use most often. This depends on your access patterns, what indexes you have, and the size of your documents. Because MongoDB uses a thread per connection model, each database connection also will need up to 1MB of RAM, whether active or idle.

If page faults are infrequent, your working set fits in RAM. If fault rates rise higher than that, you risk performance degradation. This is less critical with SSD drives than with spinning disks.

http://docs.mongodb.org/manual/faq/diagnostics/

You can use the serverStatus command to get an estimate of your current working set:

db.runCommand( { serverStatus: 1, workingSet: 1 } )