Caching Levels

There are several levels of caching used in a typical server — here they are, in order of increasing distance from the

1. Cpu cache
2. Random-Access Memory (RAM)
3. Storage controller cache
4. Storage device (disk) cache

All of these are a fixed size and set at hardware installation time, i.e. you can't move cache from one level to another. For Postgres,
there is flexibility in how #2, random-access memory, is allocated, and this provides a never-ending opportunity for administrators to
optimize their systems. The three RAM allocation possibilities are:

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