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From: James Mansion <james@mansionfamily.plus.com>
To: Mark Mielke <mark@mark.mielke.cc>
Cc: matthew@flymine.org
Cc: pgsql-performance@postgresql.org
Subject: Re: RAID arrays and performance
Date: Tue, 04 Dec 2007 15:55:30 +0000
Message-ID: <47557872.1000105@mansionfamily.plus.com> (raw)
In-Reply-To: <47557519.6090506@mark.mielke.cc>
References: <Pine.LNX.4.58.0712041213490.3731@aragorn.flymine.org>
	<87lk8aiqf3.fsf@oxford.xeocode.com>
	<Pine.LNX.4.58.0712041247540.3731@aragorn.flymine.org>
	<475559F4.7060104@mark.mielke.cc>
	<Pine.LNX.4.58.0712041349380.3731@aragorn.flymine.org>
	<4755648C.90905@mark.mielke.cc>
	<87fxyiik94.fsf@oxford.xeocode.com>
	<Pine.LNX.4.58.0712041522320.3731@aragorn.flymine.org>
	<47557519.6090506@mark.mielke.cc>

Mark Mielke wrote:
> This assumes that you can know which pages to fetch ahead of time - 
> which you do not except for sequential read of a single table.
>
Why doesn't it help to issue IO ahead-of-time requests when you are 
scanning an index?  You can read-ahead
in index pages, and submit requests for data pages as soon as it is 
clear you'll want them.  Doing so can allow
the disks and OS to relax the order in which you receive them, which may 
allow you to process them while IO
continues, and it may also optimise away some seeking and settle time.  
Maybe.

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