Cookbook recipe

How shared_buffers and the OS page cache cooperate

Applies to PostgreSQL 13–17 Last reviewed May 2026 Grounded in source

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A production experience capsule based on real incident patterns.

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Estimated investigation4 min

Scenario

You have two caches in front of disk — PostgreSQL's shared_buffers and the OS page cache. Knowing how they interact guides memory tuning. Diagnose it Check cache effectiveness: SELECT round(100*sum(blks_hit)/greatest(sum(blks_hit)+sum(blks_read),1),2) AS hit_pct FROM pg_stat_database; Why it…

Investigation Path

You have two caches in front of disk — PostgreSQL’s shared_buffers and the OS page cache. Knowing how they interact guides memory tuning.

Diagnose it

Check cache effectiveness:

SELECT round(100*sum(blks_hit)/greatest(sum(blks_hit)+sum(blks_read),1),2) AS hit_pct
FROM pg_stat_database;

Why it happens

PostgreSQL reads pages into shared_buffers; a miss there usually still hits the OS page cache (fast) before going to disk (slow). Both caches hold copies, so over-sizing shared_buffers can crowd out the OS cache.

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Concepts on this page

Shared buffers Query planner
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