From mboxrd@z Thu Jan 1 00:00:00 1970 From: Keld =?iso-8859-1?Q?J=F8rn?= Simonsen Subject: Re: (was: Re: md: raid5 vs raid10 (f2,n2,o2) benchmarks [w/10 raptors]) Date: Wed, 25 Jun 2008 21:44:18 +0200 Message-ID: <20080625194418.GC23500@rap.rap.dk> References: <20080402080206.6bd8ea65@hardcode42.net> <20080403100246.GA16459@rap.rap.dk> <20080625184838.GB21707@rap.rap.dk> Mime-Version: 1.0 Content-Type: text/plain; charset=iso-8859-1 Content-Transfer-Encoding: QUOTED-PRINTABLE Return-path: Content-Disposition: inline In-Reply-To: Sender: linux-raid-owner@vger.kernel.org To: David Lethe Cc: Justin Piszcz , "Conway S. Smith" , linux-raid@vger.kernel.org List-Id: linux-raid.ids On Wed, Jun 25, 2008 at 02:09:32PM -0500, David Lethe wrote: > -----Original Message----- > From: linux-raid-owner@vger.kernel.org [mailto:linux-raid-owner@vger.= kernel.org] On Behalf Of Keld J=F8rn Simonsen > Sent: Wednesday, June 25, 2008 1:49 PM > To: Justin Piszcz > Cc: Conway S. Smith; linux-raid@vger.kernel.org > Subject: Re: (was: Re: md: raid5 vs raid10 (f2,n2,o2) benchmarks [w/1= 0 raptors]) >=20 > > On Thu, Apr 03, 2008 at 12:02:46PM +0200, Keld J=F8rn Simonsen wrot= e: > > > On Wed, Apr 02, 2008 at 01:49:44PM -0400, Justin Piszcz wrote: > > > >=20 > > > >=20 > > > > On Wed, 2 Apr 2008, Justin Piszcz wrote: > > > >=20 > > > > > > > > > > > > > > >On Wed, 2 Apr 2008, Conway S. Smith wrote: > > >=20 > > > I have referenced both of your benchmaks in the wiki on performan= ce. So > > > now I just hope that your URLs will live forever. I also took dow= n some > > > of your recomendations there.=20 > > >=20 > > > I note that raid10,f2 has a much higher cpu load than raid10,n2 o= r > > > raid10,o2. How come? it is 31-38 % for f2, where n2 and o2 is aro= und 15 %. > > > > I found a reason for this, it seems that CPU usage and IO speed are= very > > related, so because the raid10,f2 has about double the IO performan= ce > > for sequential reading, it also has about double the cpu use. > > > > Justin's benchmark is on http://home.comcast.net/~jpiszcz/20080329-= raid/ > > > > Another of Justin's benchmarks also reveals the relation between > > IO rate and CPU use: > > http://home.comcast.net/~jpiszcz/raid/20080528/raid-levels.html > > > > Why does IO use that much CPU? Is it mostly moving around the data = from > > the kernel to the user space? Does it matter here whether one is ru= nning > > a 32 bit or a 64 bit system? >=20 > > It seems like the RAM bus can be a bottleneck. I read that DDR-400 > > can have a peak performance of 1600 MB/s. If this is halved on 32 b= it OS > > then this is 800 MB/s. And you need to both read and write when you= move > > things around. So that is 400 MB/s... And you need to still be able= to=20 > > read in from the disk controller at 330 MB/s. For 64 bit systems th= is is > > a max around 400 MB/s - given that there is a flow from the disk > > controller to system disk buffers, then from kernel buffers to user > > buffers and then from user buffers to some processing. Or am I wron= g > > here? > > > > Best regards > > Keld >=20 > These chunk sizes are profoundly meaningless if you plan on using the= m to estimate performance in the real world. The relationship between= IO rate, IO throughput, and CPU overhead will be dramatically differen= t with default md settings. Also consider that your kernel was recompi= led with little or no cpu-specific optimization, so you are wasting cpu= cycles .. and don't get me started on multicore vs. single core for su= ch benchmarks. I think something is wrong with the chunk sizes. 16 G and 7 G are most likely erroneous. Justin? David, if you have some benchmarks then please feel free to report them= =2E Best regards keld -- 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