Date: Tue, 30 Jan 2007 21:06:12 +0100
From: Andrea Arcangeli <andrea@suse.de>
To: Phillip Susi <psusi@cfl.rr.com>
Cc: Denis Vlasenko <vda.linux@googlemail.com>,
       Bill Davidsen <davidsen@tmr.com>, Michael Tokarev <mjt@tls.msk.ru>,
       Linus Torvalds <torvalds@osdl.org>, Viktor <vvp01@inbox.ru>,
       Aubrey <aubreylee@gmail.com>, Hua Zhong <hzhong@gmail.com>,
       Hugh Dickins <hugh@veritas.com>,
       Linux-kernel <linux-kernel@vger.kernel.org>
Subject: Re: O_DIRECT question
Message-ID: <20070130200612.GS8030@opteron.random>
References: <200701270035.31285.vda.linux@googlemail.com> <45BCBECE.4080405@tmr.com> <200701281803.08201.vda.linux@googlemail.com> <20070129170056.GJ8030@opteron.random> <45BE7D99.70200@cfl.rr.com> <20070130023056.GN8030@opteron.random> <45BF65E3.6070102@cfl.rr.com> <20070130164806.GQ8030@opteron.random> <45BF9381.8010108@cfl.rr.com> <20070130195720.GR8030@opteron.random>
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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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