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

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Message-ID: <20081013233933.2ee5a011@lxorguk.ukuu.org.uk>
Date:	Mon, 13 Oct 2008 23:39:33 +0100
From:	Alan Cox <alan@...rguk.ukuu.org.uk>
To:	"Grant Grundler" <grundler@...gle.com>
Cc:	"kenneth johansson" <ken@...jo.org>, linux-ide@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: weird throughput on write to SATA disk

On Mon, 13 Oct 2008 14:07:15 -0700
"Grant Grundler" <grundler@...gle.com> 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"?
> If not, could be issue with how VM writeback is (not) working.

Or just mashing up the I/O horribly. Would also be worth turning the CFQ
scheduler off or applying Arjan van de Ven's patches to fix the ioprio of
the kernel writeout threads. That last patch makes a huge difference here.

Alan
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