linux-kernel - Re: weird throughput on write to SATA disk

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Message-Id: <1223975694.3947.77.camel@duo>
Date:	Tue, 14 Oct 2008 11:14:54 +0200
From:	kenneth johansson <ken@...jo.org>
To:	Grant Grundler <grundler@...gle.com>
Cc:	linux-ide@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: weird throughput on write to SATA disk

On Mon, 2008-10-13 at 18:33 -0700, Grant Grundler wrote:
> On Mon, Oct 13, 2008 at 3:19 PM, kenneth johansson <ken@...jo.org> wrote:
> ...
> >> Can you try "dd oflag=direct if=/dev/null of=/dev/sdb bs=64k"?
> >
> > I changed to use O_DIRECT and it's much more consistent now. 69-75 with
> > 74 about 95% of the time.
> 
> That's low but it could be worse. Many things can contribute to slow
> disks. 
Well my main problem was that it was fluctuating so much that can't be
right. 

> Favorites are overtemp (See SMART field 194) and vibration (no
> measurement possible w/o special equipment). "dd" isn't exactly a
> performance application until one uses really big block sizes (1MB or
> larger).
All my numbers comes from using 1MB blocks. but I'm not using dd.
After removing the logic that actually put some data into the buffers I
do get about 105 MB/sec using O_DIRECT. I'm doing a full disk write now
to get a reference plot. Not using O_DIRECT should be almost identical. 


> > the disk is supposed to have 105 115 sustained
> > data rate.
> 
> Where did 105-115 number come from?
datasheet . they listed two drives in the same column so that was not
the range it was sustained OD and the model I have max out at 105. 


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