From owner-freebsd-current@FreeBSD.ORG  Thu Aug  4 06:03:02 2005
Return-Path: <owner-freebsd-current@FreeBSD.ORG>
X-Original-To: current@FreeBSD.ORG
Delivered-To: freebsd-current@FreeBSD.ORG
Received: from mx1.FreeBSD.org (mx1.freebsd.org [216.136.204.125])
	by hub.freebsd.org (Postfix) with ESMTP id 3054816A41F
	for <current@FreeBSD.ORG>; Thu,  4 Aug 2005 06:03:02 +0000 (GMT)
	(envelope-from ache@nagual.pp.ru)
Received: from nagual.pp.ru (nagual.pp.ru [194.87.13.69])
	by mx1.FreeBSD.org (Postfix) with ESMTP id 82FE043D46
	for <current@FreeBSD.ORG>; Thu,  4 Aug 2005 06:03:01 +0000 (GMT)
	(envelope-from ache@nagual.pp.ru)
Received: from nagual.pp.ru (ache@localhost [127.0.0.1])
	by nagual.pp.ru (8.13.4/8.13.4) with ESMTP id j7462rwh021349;
	Thu, 4 Aug 2005 10:02:53 +0400 (MSD)
	(envelope-from ache@nagual.pp.ru)
Received: (from ache@localhost)
	by nagual.pp.ru (8.13.4/8.13.4/Submit) id j7462rgc021348;
	Thu, 4 Aug 2005 10:02:53 +0400 (MSD) (envelope-from ache)
Date: Thu, 4 Aug 2005 10:02:51 +0400
From: Andrey Chernov <ache@FreeBSD.ORG>
To: Maxim.Sobolev@portaone.com
Message-ID: <20050804060251.GA21228@nagual.pp.ru>
Mail-Followup-To: Andrey Chernov <ache@freebsd.org>,
	Maxim.Sobolev@portaone.com, Dan Nelson <dnelson@allantgroup.com>,
	"current@freebsd.org" <current@FreeBSD.ORG>
References: <42F0CCD5.9090200@portaone.com>
	<20050803150117.GD93405@dan.emsphone.com>
	<42F0E9B2.9080208@portaone.com>
Mime-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <42F0E9B2.9080208@portaone.com>
User-Agent: Mutt/1.5.9i
Cc: Dan Nelson <dnelson@allantgroup.com>,
	"current@freebsd.org" <current@FreeBSD.ORG>
Subject: Re: Sub-optimal libc's read-ahead buffering behaviour
X-BeenThere: freebsd-current@freebsd.org
X-Mailman-Version: 2.1.5
Precedence: list
List-Id: Discussions about the use of FreeBSD-current
	<freebsd-current.freebsd.org>
List-Unsubscribe: <http://lists.freebsd.org/mailman/listinfo/freebsd-current>, 
	<mailto:freebsd-current-request@freebsd.org?subject=unsubscribe>
List-Archive: <http://lists.freebsd.org/pipermail/freebsd-current>
List-Post: <mailto:freebsd-current@freebsd.org>
List-Help: <mailto:freebsd-current-request@freebsd.org?subject=help>
List-Subscribe: <http://lists.freebsd.org/mailman/listinfo/freebsd-current>,
	<mailto:freebsd-current-request@freebsd.org?subject=subscribe>
X-List-Received-Date: Thu, 04 Aug 2005 06:03:02 -0000

On Wed, Aug 03, 2005 at 06:58:42PM +0300, Maxim Sobolev wrote:
> Checking the actual code confirm your point, thank you! However, I think 
> that the current behaviour is at best inconsistent, since it in fact 
> does buffering of character devices in some cases but does not in some 
> others. In my example it reads 4096 bytes instead of requested 512 at 
> each request, but when the program does fseek it rejects part of that 
> buffer and does re-positioning and re-reading. Very confusing behaviour 
> IMHO.

No surprize. Single read can be consider as sort of "atomic" operation.
But between read & seek character device itself can move its pointer in 
anyplace.

-- 
http://ache.pp.ru/