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Date: Tue, 26 Aug 1997 23:45:46 -0700 (PDT)
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To: michaelv@MindBender.serv.net
CC: tom@uniserve.com, freebsd@synertec.ene.unb.br, stable@freebsd.org
In-reply-to: <199708270529.WAA25841@MindBender.serv.net> (michaelv@MindBender.serv.net)
Subject: Re: DPT scsi card
From: asami@cs.berkeley.edu (Satoshi Asami)
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 * While that was the original "definition", RAID had become to be known
 * for more than simple redudancy in common usage.  Striping without
 * fault tolerance is a very common usage, which generally falls under
 * the term "RAID-5", if I'm not mistaken.

That's called RAID-0 according to the original paper.  And no, that is
not a really common use of the term -- when someone says "do you do
raid on your disk array?" they usually mean RAID-N where N>0.

 * From what I understand, ccd can be used for mirroring.  However,
 * recovery in case of failure is not automatic, and you can't mix
 * mirroring with striping.

I'm not sure what you mean by "mix" here, but the "mirroring" in ccd
includes striping if you have >3 disks.  I.e., you use 2N disks to get 
N times each disk's capacity.

 * So, yes, if you're using ccd to stripe for performance, you don't get
 * any redundancy features with ccd.  If you use it for mirroring, it's
 * likely that you won't get any performance benefits.

If you use it to mirror >3 disks, it will usually be faster than one
disk.  (There are some penalties on write, but I won't get into that.)

 * DPT controllers are reported to do all that stuff transparently in
 * hardware.  And, the FreeBSD driver supports basic functionality on DPT
 * cards.  However, it isn't clear how much advanced functionality the
 * FreeBSD DPT driver supports.

It has RAID-5 (parity), which just cannot be done in software
efficiently.  That is reason enough for a lot of people.  (Also, I've
heard some horror stories about data loss on software RAID-5 products
such as Sun's DiskSuite, but that's another story, as it is possible
to make those bug-free, at least in theory. :)

Satoshi