Shared buffer in postgres

Question

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?

See this lengthy thread: archives.postgresql.org/pgsql-performance/2011-05/msg00369.php — Denis de Bernardy
– Denis de Bernardy, Commented May 30, 2011 at 12:16

Guillaume Savary · Accepted Answer · 2012-10-15 12:23:47Z

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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.

answered Oct 15, 2012 at 12:23

Guillaume Savary

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Mathias Brossard · Accepted Answer · 2011-06-21 13:44:20Z

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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.

answered Jun 21, 2011 at 13:44

Mathias Brossard

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