linux-kernel - Re: O_DIRECT question

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Message-ID: <20070130200612.GS8030@opteron.random>
Date:	Tue, 30 Jan 2007 21:06:12 +0100
From:	Andrea Arcangeli <andrea@...e.de>
To:	Phillip Susi <psusi@....rr.com>
Cc:	Denis Vlasenko <vda.linux@...glemail.com>,
	Bill Davidsen <davidsen@....com>,
	Michael Tokarev <mjt@....msk.ru>,
	Linus Torvalds <torvalds@...l.org>, Viktor <vvp01@...ox.ru>,
	Aubrey <aubreylee@...il.com>, Hua Zhong <hzhong@...il.com>,
	Hugh Dickins <hugh@...itas.com>,
	Linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: O_DIRECT question

On Tue, Jan 30, 2007 at 08:57:20PM +0100, Andrea Arcangeli wrote:
> Please try yourself, it's simple enough:
> 
>        time dd if=/dev/hda of=/dev/null bs=16M count=100
>        time dd if=/dev/hda of=/dev/null bs=16M count=100 iflag=sync

sorry, reading won't help much to exercise sync ;). But the direct
line is enough to show the effect of I/O pipeline stall. To
effectively test sync of course you want to write to a file instead
(unless you want to wipe out /dev/hda ;)
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