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#11
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In article , Faeandar wrote:
Prior to this post I had never heard of raid 6 and have (still) no idea what it's structure is. Going on Malcolm's allusion I assume Double Disk Parity or Diagonal Parity or whatever is raid 6 nowadays? There is a RAID6 implementation for Linux software RAID out there (and I believe some folks are already using it in production). Perhaps the details of it would clear this up a bit. -- Joshua Baker-LePain Department of Biomedical Engineering Duke University |
#12
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In article ,
Joshua Baker-LePain wrote: In article , Faeandar wrote: Prior to this post I had never heard of raid 6 and have (still) no idea what it's structure is. Going on Malcolm's allusion I assume Double Disk Parity or Diagonal Parity or whatever is raid 6 nowadays? There is a RAID6 implementation for Linux software RAID out there (and ITYM "RAID6" HTH -- Thor Lancelot Simon But as he knew no bad language, he had called him all the names of common objects that he could think of, and had screamed: "You lamp! You towel! You plate!" and so on. --Sigmund Freud |
#13
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Benno... wrote:
Nik Simpson wrote: I think you'll find that RAID 6 is usually defined as a mechanism using two different parity/ECC schemes such that a set can survive the loss of any two disks. I stand corrected, I realised that HP had defined RAID-6 this way, but I din't realize that is a generally accepted definition. I think HP/Compaq calls this Advanced Data Guard (ADG). Isn't the RAID-6 name copyrighted to someone? You may be thinking of "RAID-7" from Storage Computer about 10 years ago. It was a glorified RAID-4 box with vocal marketing. RAID-6 is mentioned in the original Berkeley papers. They showed an NxM matrix of disks with parity across and down. As other responders have mentioned, there are ways to do N+2 directly. One method (by Blaum at IBM, US patent 5271012 + others) is "even odd", which uses diagonal parity for the second check disk. There is at least one variant of this I've seen in a paper and at least one patent application for a variant. The Rabin paper mentioned in earlier responses discusses "information dispersal", which can provide N data + M check disks. There is also a paper "Tolerating Multiple Failures in RAID Architectures with Optimal Storage and Uniform Declustering" by Guillermo Alvarez, Walter Burkhard, and Flaviu Christian of UCSD (I got it online but don't have a URL). The basic method is to define an N by N+M matrix where the first NxN elements are an identity matrix and all N+M columns are linearly independent. Encode the data by multiplying the data (in N chunks) by the matrix. Recover data by multiplying the N returned chunks (data or parity) by the inverse of the NxN matrix formed by using the columns for the chunks recovered. See also US patent 6557123 by Joseph Wiencko, assigned to Inostor. It appears to be a form of the information dispersal method, but I've just scanned the patent, not tried to read it in detail. Thomas Wicklund |
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