Cookbook recipe

The buffer manager: clock-sweep eviction

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

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Estimated investigation4 min

Scenario

When shared_buffers is full and a new page is needed, something must be evicted. The clock-sweep algorithm chooses the victim. Diagnose it Inspect buffer usage with the pg_buffercache extension: CREATE EXTENSION IF NOT EXISTS pg_buffercache; SELECT…

Investigation Path

When shared_buffers is full and a new page is needed, something must be evicted. The clock-sweep algorithm chooses the victim.

Diagnose it

Inspect buffer usage with the pg_buffercache extension:

CREATE EXTENSION IF NOT EXISTS pg_buffercache;
SELECT count(*) FROM pg_buffercache WHERE isdirty;

Why it happens

Each buffer has a usage counter. The clock sweep walks buffers, decrementing counters; a buffer with count zero and no pin is evicted (written first if dirty). Frequently-used pages keep getting re-referenced and survive.

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Related & next steps

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Concepts on this page

Backend process Shared buffers
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