Welcome to the community forums @astro!
Your first snippet from the docs is missing the opening context which is:
For the fastest processing, ensure that your indexes fit entirely in RAM so that the system can avoid reading the index from disk.
However, the must in this snippet is more correctly should . I’ll raise a DOCS pull request to fix the wording.
Your concise quote from the FAQ is also correct, but the full FAQ answer includes useful elaboration on cache size and eviction.
Both documentation pages are trying to suggest the same outcome: for best performance you will want your commonly used indexes and working set to fit in memory. This is not a strict requirement for most MongoDB storage engines (with one notable exception which I’ll mention in a moment). However, if your working set is significantly larger than available memory, performance will suffer as moving data to and from disk becomes a significant bottleneck.
The one exception to this guidance is the In-Memory Storage Engine which is part of MongoDB Enterprise edition. The In-Memory Storage Engine provides predictable latency by intentionally not maintaining any on-disk data, and requires all data to fit within the specified