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

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Date:	Mon, 13 Oct 2008 14:34:00 -0700
From:	"Grant Grundler" <grundler@...gle.com>
To:	"kenneth johansson" <ken@...jo.org>
Cc:	linux-ide@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: weird throughput on write to SATA disk

On Mon, Oct 13, 2008 at 2:22 PM, kenneth johansson <ken@...jo.org> wrote:
> 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.

Can you try "dd oflag=direct if=/dev/null of=/dev/sdb bs=64k"?

Or try with "sgp_dd"? It's part of sg3-utils package.

thanks,
grant
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