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I've noticed a significant performance drop when data is not loaded into shared_buffer when querying PostgreSQL, the difference can be almost 100 times. So in the process of optimizing the query, I was wondering if there is anyway to increase performance by increasing the shared_buffer.

Then I started to investigate the shared_buffer in PostgreSQL. and I found that the recommend value is 25% of the OS memory and PostgreSQL will take advantage of OS cache to accelerate the query. But from what I've seen with my own db, reading from disk vs shared_buffer has huge difference, so I would like to query from shared_buffer for the most time.

So I wondered, what's the downside if I increase the shared_buffer in PostgreSQL? What if I only increase the shared_buffer in my readonly instance?

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A downside of increasing the buffer cache is double buffering. When you need to read a page into shared_buffers, it might first need to evict an existing page to make room for it. But then the OS cache might need to evict a page from itself as well so to make room for it to read the page from the actual disk. Then you end up with the same page being located in both places, which wastes cache space. So then instead of reading a page from the OS cache you are more likely to need to read it from actual disk, which is far slower. From a double-buffering perspective, you probably want shared_buffers to be much less than half of the system RAM (using OS cache as the main cache) or much larger than half (using shared_buffers as the main cache)

Another downside is that if it is too large, you might start to get out-of-memory errors or invoke the OOM killer or otherwise destabilize the system.

Another problem is that after some operations, like DROP TABLE, TRUNCATE, or the ending of a COPY in some circumstances, PostgreSQL needs to invalidate a lot of buffers and chooses to do so by scouring the entire buffer cache. If you do a lot of those operations, that time can really add up with large buffer cache settings.

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Do you have experience with AWS RDS PostgreSQL? I am not sure whether I should choose shared buffer or os cache as the main cache in AWS RDS.
I don't see this as being an issue if we have a lot of unused memory (100GB or 70% say).
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Some workloads (I know about DROP TABLE, but there may be others) perform better with a smaller shared_buffers. But essentially, it is a matter of trial and error (or better yet: reproducible performance tests).

If you can make shared_buffers big enough that it can hold everything you need from the database, that is probably a good choice.

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