From mboxrd@z Thu Jan 1 00:00:00 1970 From: Goswin von Brederlow Subject: Re: Random IO with md raid Date: Wed, 16 Dec 2009 12:32:20 +0100 Message-ID: <87zl5j6tob.fsf@frosties.localdomain> References: <4B198DBA.5000906@matws.net> <4B1A6C28.20800@shiftmail.org> <4B1D88D2.9000001@matws.net> <87hbs03y0t.fsf@frosties.localdomain> <4B260000.3040703@matws.net> <4877c76c0912140213u3dab2fcfxfd7ed1b4ff79f4ad@mail.gmail.com> Mime-Version: 1.0 Content-Type: text/plain; charset=iso-8859-1 Content-Transfer-Encoding: QUOTED-PRINTABLE Return-path: In-Reply-To: <4877c76c0912140213u3dab2fcfxfd7ed1b4ff79f4ad@mail.gmail.com> (Michael Evans's message of "Mon, 14 Dec 2009 02:13:39 -0800") Sender: linux-raid-owner@vger.kernel.org To: Michael Evans Cc: Matthieu Patou , Goswin von Brederlow , Asdo , linux-raid List-Id: linux-raid.ids Michael Evans writes: > On Mon, Dec 14, 2009 at 1:06 AM, Matthieu Patou wrote= : >> Hello Goswin, >>> Software raid1 will write twice as much data. That means twice as m= uch >>> data goes over the system bus and into the controler cache. >>> Effectively you have halve the cache size. Maybe that is all you se= e. >>> >>> MfG >>> =A0 =A0 =A0 =A0 =A0Goswin >>> >> Your idea seems logical, and I took a few hours today to verify it a= nd it's >> the case as accessing the disk without software raid leads to almost= the >> same result as with hardware raid. >> >> Matthieu. >> -- >> To unsubscribe from this list: send the line "unsubscribe linux-raid= " in >> the body of a message to majordomo@vger.kernel.org >> More majordomo info at =A0http://vger.kernel.org/majordomo-info.html >> > > Thinking about it, the doubling of data going over the system bus par= t > is correct. You must still push the operation out to each device's > buffer. However for -most- modern systems the system-bus will not be > the bottleneck for a reasonable number of drives. > > Further the later half which I'd skimmed over the first time is > utterly incorrect. Each drive would still only see the commands > targeting that drive. That should be virtually the same if not > identical to the single-drive case. The controler itself has cache. And that is shared between all drives. > The two most likely bottlenecks are single-drive write speed, and any > IO barriers that are used to ensure file system consistency in the > event of sudden interruption. MfG Goswin -- To unsubscribe from this list: send the line "unsubscribe linux-raid" i= n the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html