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From: Gerardo Herzig <[email protected]>
To: Soni M <[email protected]>
Cc: [email protected]
Subject: Re: OS cache management
Date: Wed, 6 Sep 2017 14:13:34 +0000 (UTC)
Message-ID: <[email protected]> (raw)
In-Reply-To: <CAAMgDXkDgmnnzdDtk1pOX473eLK=eQN+MnccncGHiNBAtiRnTw@mail.gmail.com>
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----- Mensaje original -----
> De: "Soni M" <[email protected]>
> Para: [email protected]
> Enviados: Miércoles, 6 de Septiembre 2017 5:12:26
> Asunto: [PERFORM] OS cache management
> 
> Hello All, I would like to know about how OS cache works for postgres table
> and index file.
> 
> Let's say I have 10 year data, and commonly used data only the last 1 year.
> This data is quite big, so each table and index file is divided into
> several file in PGDATA/base
> 
> Let's say 1 index named order_by_date has relfilenode = 1870772348, and
> it's file consist of 1870772348, 1870772348.1, and 1870772348.2
> 
> And for oftenly queried 1 year data, do ALL files for the order_by_date
> pushed to OS cache ? or it's just 1 file that contains index to this 1 year
> data.
> 

Postgres has its own cache (defined by the "shared_buffers" variable). Usually, the unit of movement in and out from the cache is a 8k page (defined at compilation time), so you cant put it directly in terms of files.

There is an extension that can inspect the cache contents:
https://www.postgresql.org/docs/current/static/pgbuffercache.html

HTH
Gerardo


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