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

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Message-Id: <1223932956.3947.22.camel@duo>
Date:	Mon, 13 Oct 2008 23:22:36 +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 14:07 -0700, Grant Grundler wrote:
> On Mon, Oct 13, 2008 at 12:19 PM, kenneth johansson <ken@...jo.org> wrote:
> > When writing data to /dev/sdb (that is the whole disk and no filesystem)
> > it starts out ok around 100MB/sec but soon end up doing considerably
> > worse. For prolonged times lasting several minutes it hovers in the
> > 20-30MB/sec.
> 
> Is the application using "O_DIRECT"?

no just open then write no fancy stuff

> If not, could be issue with how VM writeback is (not) working.

the access pattern is the simplest possible. Is there any knobs I could
try to adjust.

> Can you reproduce this using fio or even "dd"?
> (See git://git.kernel.dk/fio)

dd is having the same effect. 


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