From mboxrd@z Thu Jan 1 00:00:00 1970 From: Michael Tokarev Subject: Re: Unbelievably _good_ write performance: RHEL5.4 mirror Date: Wed, 04 Nov 2009 15:40:47 +0300 Message-ID: <4AF1764F.20302@msgid.tls.msk.ru> References: <921ca19c0911030437s4ba6b2d9l24fe4d2c83029e9b@mail.gmail.com> <4877c76c0911030831q2ca99007lf3b9068de6be48b@mail.gmail.com> <4877c76c0911031611h3572851avaed9870795db3d92@mail.gmail.com> <4877c76c0911031629n580dc25ay7e47990908b056e7@mail.gmail.com> Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 7bit Return-path: In-Reply-To: <4877c76c0911031629n580dc25ay7e47990908b056e7@mail.gmail.com> Sender: linux-raid-owner@vger.kernel.org To: Michael Evans Cc: Chris Worley , Sujit K M , linuxraid List-Id: linux-raid.ids Michael Evans wrote: >> So, you're saying that the MD layer is doing it's own buffering? Are >> you sure? With the system cache disabled and the drive (and block >> device driver) cache disabled, there should be no reason to require >> synchronous I/O, unless, as you suggest, the MD layer is broken. >> >> You're saying that both O_DIRECT and O_SYNC must be used to disable >> cache effects. Why then are there two separate flags and not just one? >> Synchronous is a very different behavior that is not necessary for >> this test and put additional requirements that are not needed for this >> test. >> >> Chris >> > > Reading the manual page it seems O_DIRECT explicitly minimizes any > attempts at extra copying; not explicitly disabling buffers, merely > not adding more. In another mail thread this tweak-able was > discussed: > > echo 0 > /sys/block/md*/md/stripe_cache_size stripe_cache_size only exists for raid4, raid5 and raid6. It never existed for other raid levels because it's pointless. /mjt