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List:       beowulf
Subject:    RE: [Beowulf] Storage - the end of RAID?
From:       "Lux, Jim (337C)" <james.p.lux () jpl ! nasa ! gov>
Date:       2010-10-29 20:40:27
Message-ID: ECE7A93BD093E1439C20020FBE87C47FEDD2AEE07E () ALTPHYEMBEVSP20 ! RES ! AD ! JPL
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> -----Original Message-----
> From: beowulf-bounces@beowulf.org [mailto:beowulf-bounces@beowulf.org] On Behalf Of \
>                 Joe Landman
> Sent: Friday, October 29, 2010 1:10 PM
> To: beowulf@beowulf.org
> Subject: Re: [Beowulf] Storage - the end of RAID?
> 
> RAID IS NOT A BACKUP (can't say how many times I've had to say this to
> customers).  It can (and does) occasionally fail.  The only *guaranteed*
> way to prevent the failure from increasing entropy significantly in the
> universe is to have a recent copy of all the relevant data.
> 
> Which is RAID1 all over again.
> 

Could one not design a coding strategy that uses a bit more redundancy than the (8,3) \
Huffman code and that essentially doesn't need to be rebuilt.. that is, say you have \
12 drives to store 8 drives worth of data, and for ease of talking, one bit/byte is \
written across the array.

A drive fails, and you can still read the data ok from the remaining 11 (and, in \
fact, tolerate another failure). You put in a new drive, which contains all "wrong" \
bits (actually, half the bits are wrong and half are right, but you don't know which \
are which)...

You read from the full array, and the bits that are wrong on the new array just get \
corrected during the read in the usual way.  You write to the array, and all 12 bits \
get written.  So gradually, the new drive gets filled with "correct" bits.  As long \
as you don't get another TWO failures before all the bits are ok, you're in good \
shape.

(yeah, you could do writeback on error to fill in erroneous bits in the background, \
etc. but I assume that's not an option because of performance).

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