linux-kernel - Re: Very high IOWait during all disk activity

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Message-ID: <74d8fc520802290747o2c97992ar7af2b8c295fe2fb2@mail.gmail.com>
Date:	Fri, 29 Feb 2008 15:47:08 +0000
From:	"Gordon Mckeown" <ml_lkml@...fluffyone.net>
To:	LKML <linux-kernel@...r.kernel.org>
Cc:	"Tomasz Chmielewski" <mangoo@...g.org>
Subject: Re: Very high IOWait during all disk activity

On Fri, Feb 29, 2008 at 3:18 PM, Tomasz Chmielewski <mangoo@...g.org> wrote:
>  Unless you can write to the disk faster than fetch data from /dev/zero -
>  yes, it is normal.

OK, thank you; it has been a struggle to get confirmation of this;
perhaps because the way IOWait is measured has changed at some point?

>  Try running:
>
>    cat /dev/zero | bzip2 -c >/dev/null
>
>  when your IOwait is big (because you write a big file), and then watch
>  the numbers.

Ah, I can see that the CPU-intensive command soaks up the cycles that
would otherwise have been reported as IOWaits.

Unfortunately this doesn't help explain why Windows XP on the same box
can complete an "identical" copy operation in half the time. Perhaps
it's just due to the different filesystems, or the way write caching
works?

Thanks,

Gordon.
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