From mboxrd@z Thu Jan 1 00:00:00 1970 From: Keld =?iso-8859-1?Q?J=F8rn?= Simonsen Subject: Re: Performance question, RAID5 Date: Mon, 31 Jan 2011 22:41:13 +0100 Message-ID: <20110131214113.GA5274@www2.open-std.org> References: <20110130094444.68288b0e@natsu> <20110130171533.4c9e236b@natsu> <4D45C3FA.2040900@hardwarefreak.com> <20110131085202.GA25912@www2.open-std.org> <20110131131131.GA26525@www2.open-std.org> <20110201074257.3c771181@notabene.brown> 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: <20110201074257.3c771181@notabene.brown> Sender: linux-raid-owner@vger.kernel.org To: NeilBrown Cc: Keld =?iso-8859-1?Q?J=F8rn?= Simonsen , Mathias =?iso-8859-1?Q?Bur=E9n?= , Stan Hoeppner , Roman Mamedov , CoolCold , Linux-RAID List-Id: linux-raid.ids On Tue, Feb 01, 2011 at 07:42:57AM +1100, NeilBrown wrote: > On Mon, 31 Jan 2011 14:11:31 +0100 Keld J=F8rn Simonsen wrote: >=20 > > On Mon, Jan 31, 2011 at 09:37:46AM +0000, Mathias Bur=E9n wrote: > > > On 31 January 2011 08:52, Keld J=F8rn Simonsen = wrote: > > > > If your intallation is CPU bound, and you are > > > > using an Atom N270 processor or the like, well some ideas: > > > > > > > > The Atom CPU may have threading, so you could run 2 RAIDs > > > > which then probably would run in each thread. > > > > It would cost you 1 more disk if you run 2 RAID5's > > > > so you get 8 TB payload out of your 12 GB total (6 drives of 2 = TB each). > > > > > > > > Another way to get better performance could be to use less > > > > CPU-intensitive RAID types. RAID5 is intensitive as it needs to > > > > calculate XOR information all the time. Maybe a mirrored > > > > raid type like RAID10,f2 would give you less CPU usage, > > > > and the run 2 RAIDS to have it running in both hyperthreads. > > > > Here you would then only get 6 TB payload of your 12 GB disks, > > > > but then also probably a faster system. > > > > > > > > Best regards > > > > keld > > > > > > >=20 > > > Hi, > > >=20 > > > It's interesting what you say about the XOR calculations. I thoug= ht > > > that it was only calculated on writes? The Atom (330) has HT, so = Linux > > > sees 4 logical CPUs. > >=20 > > Yes you are right, it only calculates XOR on writes with RAID5.=20 > > But then I am puzzled what all these CPU cycles are used for. > > Also many cycles are used on mirrored raid types. Why? > > Maybe some is because of LVM? I have been puzzled for a long time w= hy > > ordinary RAID without LVM need to use so much CPU. Maybe a lot of d= ata > > sguffling between buffers? Neil? >=20 > What is your evidence that RAID1 uses lots of CPU? Much of this is raid10, but it should be the same: http://home.comcast.net/~jpiszcz/20080329-raid/ http://home.comcast.net/~jpiszcz/raid/20080528/raid-levels.html It seems like cpu usage is rather proportionate to the IO done. And the CPU usage does get up to about 40 % for reading, and 45 % for writing - this is most likely a significant delay factor. =46or slower CPUs like the Atom CPU this may be an even more significan= t delay factor. =46or RAID5 the situation is even worse, as expected. 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