From mboxrd@z Thu Jan 1 00:00:00 1970 From: Bill Davidsen Subject: Re: RAID10 Layouts Date: Fri, 21 Aug 2009 17:57:43 -0400 Message-ID: <4A8F1857.2010305@tmr.com> References: <200908210627.06241.Info@quantum-sci.net> <87ljldce33.fsf@frosties.localdomain> <20090821204234.GA3683@rap.rap.dk> Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: QUOTED-PRINTABLE Return-path: In-Reply-To: <20090821204234.GA3683@rap.rap.dk> Sender: linux-raid-owner@vger.kernel.org To: =?ISO-8859-1?Q?Keld_J=F8rn_Simonsen?= Cc: Goswin von Brederlow , Info@quantum-sci.net, linux-raid@vger.kernel.org List-Id: linux-raid.ids Keld J=F8rn Simonsen wrote: > On Fri, Aug 21, 2009 at 06:43:28PM +0200, Goswin von Brederlow wrote: > =20 >> Info@quantum-sci.net writes: >> >> =20 >>> Hello list, >>> >>> Researching RAID10, trying to learn the most advanced system for a = 2 >>> SATA drive system. Have two WD 2TB drives for a media computer, an= d >>> the most important requirement is data redundancy. I realize that >>> RAID is no substitute for backups, but this is a backup for the >>> backups and the purpose here is data safety. The secondary goal is >>> speed enhancement. It appears that RAID10 can give both. >>> >>> First question is on layout of RAID10. In studying the man pages i= t >>> seems that Far mode gives 95% of the speed of RAID0, but with >>> increased seek for writes. And that Offset retains much of this >>> benefit while increasing efficiency of writes. What should be the >>> preference, Far or Offset? Are they equally as robust? >>> =20 >> All raid10 layouts offer the same robustness. Which layout is best f= or >> you really depends on your use case. Probably the biggest factor wil= l >> be the average file size. My experience is that with large files the >> far copies do not cost noticeable write speed while being twice as >> fast reading as raid1. >> =20 > > The file system elevator makes up for the Far write head movement. > > =20 >>> How safe is the data in Far or Offset mode? If a drive fails, will >>> a complete, usable, bootable system exist on the other drive? >>> (These two are the only drives in the system, which is Debian >>> Testing, Debian kernel 2.6.30-5) Need I make any special Grub >>> settings? >>> =20 >> I don't think lilo or grub1 can boot from raid10 at all with offset = or >> far copies. With near copies you are identical to a simple raid1 so >> that would boot. >> =20 > > there is a howto on setting up a system, that can continue runnig, if= one=20 > disk fails at > http://linux-raid.osdl.org/index.php/Preventing_against_a_failing_dis= k > > =20 >>> How does this look: >>> # mdadm --create /dev/md0 --level=3Draid10 --layout=3Do2 --metadata= =3D1.2 --chunk=3D64 --raid-disks=3D2 missing /dev/sdb1 >>> =20 >> On partitions it is save to use 1.1 format. Saves you 4k. Jupey. >> >> You should play with the chunksize though and try with and without >> bitmap and different bitmap sizes. Bitmap costs some write performan= ce >> but it greatly speeds up resyncs after a crash or temporary drive >> failure. >> =20 > > I would recommend a bigger chunk size. at least 256 kiB. > =20 You really want to look at stripe-size and stride-size creating an=20 ext[234] filesystem on top of raid, good things to happen there. --=20 bill davidsen CTO TMR Associates, Inc "You are disgraced professional losers. And by the way, give us our mon= ey back." - Representative Earl Pomeroy, Democrat of North Dakota on the A.I.G. executives who were paid bonuses after a federal bailout= =2E -- 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