RE: Yet another RAID Question (YARQ)
From: Ted Mittelstaedt (tedm_at_toybox.placo.com)
Date: 06/20/05
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To: "Sandy Rutherford" <sandy@krvarr.bc.ca>, "Alex Zbyslaw" <xfb52@dial.pipex.com> Date: Mon, 20 Jun 2005 01:11:54 -0700
>-----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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