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From: James Mansion <james@mansionfamily.plus.com>
To: Robert Haas <robertmhaas@gmail.com>
Cc: Claudio Freire <klaussfreire@gmail.com>
Cc: Tomas Vondra <tv@fuzzy.cz>
Cc: pgsql-performance@postgresql.org
Subject: Re: Performance
Date: Fri, 29 Apr 2011 09:25:00 +0100
Message-ID: <4DBA75DC.6070506@mansionfamily.plus.com> (raw)
In-Reply-To: <D2F0DB51-7D42-4573-99CA-B72E28440B16@gmail.com>
References: <DF2D5436-117C-4D02-9B3C-A55723B7DDE1@darkstatic.com>
	<20110412171855.GA14292@tux>
	<FC3A3A2B-3ECB-41BA-8F94-356D6FED3695@darkstatic.com>
	<4DA496FA.3070908@fuzzy.cz>
	<8F22D592-23C1-4A3C-94A5-48363332ADD3@darkstatic.com>
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	<B93DFBBB-DA56-4044-A508-0B7E4A2CFD28@darkstatic.com>
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	<A0B9339E-9D58-4842-A206-050B773360B6@darkstatic.com>
	<4DA56DA8020000250003C783@gw.wicourts.gov>
	<BANLkTinJqXqErAtQg--1MOaw2LJNFZmwOg@mail.gmail.com>
	<BANLkTimyWkoX8Dj=4CKAjhY82ibru-An7g@mail.gmail.com>
	<4DA6216E.9020907@fuzzy.cz>
	<BANLkTim_m-oz3UMXEKUdFjWJ6uuAza79cw@mail.gmail.com>
	<4DA63125.5070106@fuzzy.cz>
	<BANLkTikYtnzTfS8Yjc-3ap9kysXLLpJwmg@mail.gmail.com>
	<D2F0DB51-7D42-4573-99CA-B72E28440B16@gmail.com>

Robert Haas wrote:
> The server can and does measure hit rates for the PG buffer pool, but to my knowledge there is no clear-cut way for PG to know whether read() is satisfied from the OS cache or a drive cache or the platter.
>
>   
Does the server know which IO it thinks is sequential, and which it 
thinks is random?  Could it not time the IOs (perhaps optionally) and at 
least keep some sort of statistics of the actual observed times?

It might not be appropriate for the server to attempt auto-tuning, but 
it might be able to provide some information that can be used by a DBA 
to make informed decisions.

James




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