From owner-freebsd-hackers  Tue Mar  5 13: 0:31 2002
Delivered-To: freebsd-hackers@freebsd.org
Received: from rwcrmhc53.attbi.com (rwcrmhc53.attbi.com [204.127.198.39])
	by hub.freebsd.org (Postfix) with ESMTP id 4B15D37B402
	for <freebsd-hackers@freebsd.org>; Tue,  5 Mar 2002 13:00:17 -0800 (PST)
Received: from InterJet.elischer.org ([12.232.206.8])
          by rwcrmhc53.attbi.com
          (InterMail vM.4.01.03.27 201-229-121-127-20010626) with ESMTP
          id <20020305210016.SKDF2951.rwcrmhc53.attbi.com@InterJet.elischer.org>;
          Tue, 5 Mar 2002 21:00:16 +0000
Received: from localhost (localhost.elischer.org [127.0.0.1])
	by InterJet.elischer.org (8.9.1a/8.9.1) with ESMTP id MAA32421;
	Tue, 5 Mar 2002 12:55:30 -0800 (PST)
Date: Tue, 5 Mar 2002 12:55:30 -0800 (PST)
From: Julian Elischer <julian@elischer.org>
To: Zhihui Zhang <zzhang@cs.binghamton.edu>
Cc: freebsd-hackers@freebsd.org
Subject: Re: A weird disk behaviour
In-Reply-To: <Pine.SOL.4.21.0203051527280.10139-100000@onyx>
Message-ID: <Pine.BSF.4.21.0203051255050.26829-100000@InterJet.elischer.org>
MIME-Version: 1.0
Content-Type: TEXT/PLAIN; charset=US-ASCII
Sender: owner-freebsd-hackers@FreeBSD.ORG
Precedence: bulk
List-ID: <freebsd-hackers.FreeBSD.ORG>
List-Archive: <http://docs.freebsd.org/mail/> (Web Archive)
List-Help: <mailto:majordomo@FreeBSD.ORG?subject=help> (List Instructions)
List-Subscribe: <mailto:majordomo@FreeBSD.ORG?subject=subscribe%20freebsd-hackers>
List-Unsubscribe: <mailto:majordomo@FreeBSD.ORG?subject=unsubscribe%20freebsd-hackers>
X-Loop: FreeBSD.ORG


more writes fit in the disk's write cache?


On Tue, 5 Mar 2002, Zhihui Zhang wrote:

> 
> I am doing some raw I/O test on a seagate SCSI disk running FreeBSD 4.5.
> This situation is like this:
> 
>  +-----+----+----+----+----+----+----+----+----+----+---+------
>  |     |    |    |    |    |    |    |    |    |    |   | ....
>  +-----+----+----+----+----+----+----+----+----+----+---+------
> 
> Each block is of fixed size, say 8192 bytes. Now I have a user program
> writing each contiguously laid out block sequentially using /dev/daxxx
> interface. There are a lot of them, say 15000.  I write the blocks in two
> ways (the data used in writing are garbage):
> 
> (1) Write each block fully and sequentially, ie. 8192 bytes.
> 
> (2) I still write these blocks sequentially, but for each block I only
> write part of it.  Exactly how many bytes are written inside each block is
> determinted by a random number between 512 .. 8192 bytes (rounded up a
> to multiple of 512 bytes).
> 
> I find out the the performance of (2) is several times better than the
> performance of (1). Can anyone explain to me why this is the case?
> 
> Thanks for any suggestions or hints.
> 
> -Zhihui
> 
> 
> 
> To Unsubscribe: send mail to majordomo@FreeBSD.org
> with "unsubscribe freebsd-hackers" in the body of the message
> 


To Unsubscribe: send mail to majordomo@FreeBSD.org
with "unsubscribe freebsd-hackers" in the body of the message