Date: Tue, 26 Aug 1997 23:45:46 -0700 (PDT) From: asami@cs.berkeley.edu (Satoshi Asami) To: michaelv@MindBender.serv.net Cc: tom@uniserve.com, freebsd@synertec.ene.unb.br, stable@freebsd.org Subject: Re: DPT scsi card Message-ID: <199708270645.XAA13256@silvia.HIP.Berkeley.EDU> In-Reply-To: <199708270529.WAA25841@MindBender.serv.net> (michaelv@MindBender.serv.net)
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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
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