CODEWITHSUNDEEP
postgresql
pree
pree

Reputation: 2357

Shared buffer in postgres

I am curious about the role played by shared buffer in postgres. Shared buffer maintains all the recently accessed disk pages and dirty pages. If a new page needs to be brought in and there is no space left in shared buffer, a victim dirty page is written back to the disk.

However, I am confused about this statement- PostgreSQL depends on the OS for caching. (http://www.varlena.com/GeneralBits/Tidbits/perf.html#shbuf)"

How does postgres depends on the OS for caching? And how does it change the behavior of shared buffer?

Upvotes: 9

Views: 14174

Answers (2)

Guillaume Savary
Guillaume Savary

Reputation: 81

Postgresql uses the OS cache and its own data cache. The two are useful according to your database usage.

OS cache is very fast but basic: it removes older data with the new one. It is useful for very versatile query results. PG cache is slower (still much faster than disk) but it keeps usage counters of the most used data. Useful for recurrent results/index.

Upvotes: 8

Mathias Brossard
Mathias Brossard

Reputation: 3786

I think this link is clearer (and more up-to-date).

My understanding is that shared_buffers is where PostgreSQL processes work and share information, but above a certain limit (15% to 25% of the server RAM) diminishing returns makes it more interesting to leave more RAM to the OS to perform caching itself.

Upvotes: 5

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