From owner-freebsd-hackers@FreeBSD.ORG  Wed Jul  8 15:54:39 2009
Return-Path: <owner-freebsd-hackers@FreeBSD.ORG>
Delivered-To: freebsd-hackers@freebsd.org
Received: from mx1.freebsd.org (mx1.freebsd.org [IPv6:2001:4f8:fff6::34])
	by hub.freebsd.org (Postfix) with ESMTP id 655471065674;
	Wed,  8 Jul 2009 15:54:39 +0000 (UTC)
	(envelope-from wojtek@wojtek.tensor.gdynia.pl)
Received: from wojteks.tensor.gdynia.pl (wojteks.tensor.gdynia.pl
	[IPv6:2001:4070:101:2::1])
	by mx1.freebsd.org (Postfix) with ESMTP id 73A808FC15;
	Wed,  8 Jul 2009 15:54:37 +0000 (UTC)
	(envelope-from wojtek@wojtek.tensor.gdynia.pl)
Received: from wojtek.tensor.gdynia.pl (wojtek.tensor.gdynia.pl
	[IPv6:2001:4070:101:2::2])
	by wojteks.tensor.gdynia.pl (8.14.3/8.14.3) with ESMTP id
	n68FtBSM050761; Wed, 8 Jul 2009 17:55:11 +0200 (CEST)
	(envelope-from wojtek@wojtek.tensor.gdynia.pl)
Received: from wojtek.tensor.gdynia.pl (localhost [127.0.0.1])
	by wojtek.tensor.gdynia.pl (8.14.3/8.14.3) with ESMTP id n68Ft8Qp001940;
	Wed, 8 Jul 2009 17:55:08 +0200 (CEST)
	(envelope-from wojtek@wojtek.tensor.gdynia.pl)
Received: from localhost (wojtek@localhost)
	by wojtek.tensor.gdynia.pl (8.14.3/8.14.3/Submit) with ESMTP id
	n68Ft8ak001937; Wed, 8 Jul 2009 17:55:08 +0200 (CEST)
	(envelope-from wojtek@wojtek.tensor.gdynia.pl)
Date: Wed, 8 Jul 2009 17:55:07 +0200 (CEST)
From: Wojciech Puchar <wojtek@wojtek.tensor.gdynia.pl>
To: Ivan Voras <ivoras@freebsd.org>
In-Reply-To: <h3209m$dog$1@ger.gmane.org>
Message-ID: <alpine.BSF.2.00.0907081749100.1920@wojtek.tensor.gdynia.pl>
References: <995877b70907080042he7aee18m22f9d4a0e4b18bef@mail.gmail.com>
	<h3209m$dog$1@ger.gmane.org>
User-Agent: Alpine 2.00 (BSF 1167 2008-08-23)
MIME-Version: 1.0
Content-Type: TEXT/PLAIN; charset=US-ASCII; format=flowed
Cc: freebsd-hackers@freebsd.org
Subject: Re: Regarding IO to disk
X-BeenThere: freebsd-hackers@freebsd.org
X-Mailman-Version: 2.1.5
Precedence: list
List-Id: Technical Discussions relating to FreeBSD
	<freebsd-hackers.freebsd.org>
List-Unsubscribe: <http://lists.freebsd.org/mailman/listinfo/freebsd-hackers>, 
	<mailto:freebsd-hackers-request@freebsd.org?subject=unsubscribe>
List-Archive: <http://lists.freebsd.org/pipermail/freebsd-hackers>
List-Post: <mailto:freebsd-hackers@freebsd.org>
List-Help: <mailto:freebsd-hackers-request@freebsd.org?subject=help>
List-Subscribe: <http://lists.freebsd.org/mailman/listinfo/freebsd-hackers>,
	<mailto:freebsd-hackers-request@freebsd.org?subject=subscribe>
X-List-Received-Date: Wed, 08 Jul 2009 15:54:39 -0000

>
> Declared average latency for desktop hard drives (see 
> http://www.seagate.com/docs/pdf/datasheet/disc/ds_barracuda_7200_12.pdf) is

latency is average time to wait until data will be below head. it's by 
average half on single rotation. for 7200 rpm drive it's 120rps so half 
rotation takes 1/240 second=about 4ms.

But to perform I/O you have to

- send a request to disk (<1ms)
- move head - depends of distance, up to 20-30ms, 8-10ms average on most 
drives
- wait for latency - 4ms average.

you end in 13-15ms average.

in case of multiple parallel accesses - drive accept writes immediatelly, 
and can accept multiple reads if it can NCQ/TCQ, then it internally sort 
request queue to minimize required head moves and rotational waiting, so 
under high load you may go to 6-7ms/request which is about 150IOPS.

Of course i'm talking about random/pseudorandom reads, multiple reads of 
sequential addresses will be optimized by drive.

One more - i'm talking about raw I/O, FreeBSD of course optimizes 
filesystem acces internally by using lots of caching (as much as available 
memory) and planning requests.

small writes to file usually succeed immediatelly (to cache).