Date: Mon, 20 Jun 2005 01:11:54 -0700 From: "Ted Mittelstaedt" <tedm@toybox.placo.com> To: "Sandy Rutherford" <sandy@krvarr.bc.ca>, "Alex Zbyslaw" <xfb52@dial.pipex.com> Cc: "P.U.Kruppa" <root@pukruppa.de>, freebsd-questions@freebsd.org Subject: RE: Yet another RAID Question (YARQ) Message-ID: <LOBBIFDAGNMAMLGJJCKNGEMAFBAA.tedm@toybox.placo.com> In-Reply-To: <17078.22937.726253.34028@szamoca.krvarr.bc.ca>
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>-----Original Message----- >From: owner-freebsd-questions@freebsd.org >[mailto:owner-freebsd-questions@freebsd.org]On Behalf Of Sandy >Rutherford >Sent: Sunday, June 19, 2005 10:52 PM > >In order to boost read performance, a RAID card should interleave >reading from a RAID-1 volume by reading alternately from one drive and >then the other. You can see this in alternate blinking of the >activity lights of the drives. If you are not seeing this when >copying a large file, then this would suggest that a RAID-1 volume is >not working as it should. > Incorrect. What you are describing is RAID-0. RAID-1 is mirroring. Here's a link: http://www.pcguide.com/ref/hdd/perf/raid/index.htm Here's the different RAID levels: RAID-0 interleaving between 2 or more disks. Primary purpose is to combine multiple disks into a larger volume. Gives maximum amount of space with no fault tolerance. RAID-1 mirroring Requires pairs of disks. Primary purpose is to give fault tolerance. Most commonly used with cheaper IDE disks and IDE RAID cards. Uses fewest number of disk drives for fault tolerance. Very easy to design so that if 1 disk dies the array of disks continues without interruption RAID-2 Bit level striping. Not used in modern systems (the scheme was overengineered, basically) RAID-3 Byte level striping. Rarely seen in modern RAID controllers. RAID-4 Block level striping. Rarely seen in modern RAID controllers. RAID-5 Block level striping - with distributed parity. Requires a minimum of 3 disks. The primary purpose is to give the volume-combining features of RAID-0 with the redundancy of RAID-1. This is the most popular RAID. But it is more difficult to design for so the cheaper controllers sometimes will halt the system if a disk is lost. Also requires drivers in the OS to allow online rebuilding of a replaced disk drive. Requires significant CPU processing on the RAID card for parity calculations. RAID-6 Same as RAID-5 except parity is dual distributed, not single distributed. Not common althogh some manufacturers call their proprietary extensions to RAID-5, "raid 6" RAID-7 Patented RAID solution of Storage Computer Corporation that first showed up in their OmniRAID stuff, now seen in their CyberBorgVSA. (influence of Star Trek in the product name, there) Ted Mittelstaedt Author, The FreeBSD Corporate Networker's Guide http://www.freebsd-corp-net-guide.com
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