Date: Thu, 15 Jan 2015 17:10:31 +0000 (UTC)
From: Glyn Astill <glynastill@yahoo.co.uk>
Reply-To: Glyn Astill <glynastill@yahoo.co.uk>
To: Huan Ruan <huan.ruan.it@gmail.com>,
 "pgsql-performance@postgresql.org" <pgsql-performance@postgresql.org>
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Subject: Re: shared_buffers vs Linux file cache
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> From: Huan Ruan <huan.ruan.it@gmail.com>
>To: pgsql-performance@postgresql.org 
>Sent: Thursday, 15 January 2015, 11:30
>Subject: [PERFORM] shared_buffers vs Linux file cache
> 
>
>
>Hi All
>
>
>I thought 'shared_buffers' sets how much memory that is dedicated to PostgreSQL to use for caching data, therefore not available to other applications.
>
>
>However, as shown in the following screenshots, The server (CentOS 6.6 64bit) has 64GB of RAM, and 'shared_buffer' is set to 32GB, but the free+buffer+cache is 60GB. 
>
>
>Shouldn't the maximum value for free+buffer+cache be 32GB ( 64 - 32)?
>Is 'shared_buffers' pre allocated to Postgres, and Postgres only?

>

I've not looked at the images, but I think you're getting PostgreSQL shared_buffers and the OS buffercache mixed up; they are not the same.

PostgreSQL shared_buffers is specific to postgres, whereas the OS buffercache will just use free memory to cache data pages from disk, and this is what you're seeing.

Some reading for you: 
http://www.tldp.org/LDP/sag/html/buffer-cache.html

Glyn


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