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

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Date:	Mon, 13 Oct 2008 18:33:55 -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 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. 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). sgp_dd is better since it is multi-threaded. fio has all the
right knobs to test disk perf and measure it properly.

> the disk is supposed to have 105 115 sustained
> data rate.

Where did 105-115 number come from?

The best I've seen to date with 7200 rpm drives was 108MB/s on the
outside diameter. That degrades slowly until about 60-70% towards the
inside diameter and then drops off dramatically (down to something
like 50MB/s).

hth,
grant
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