RE: RAID-6: help wanted

I took a look at your paper.  It's magic!  :)
My head hurts. :(

Ok, I understand that 27 disks is the limit.  But is that usable?  Would it
be too slow?  The disk IO seems unreasonable with 27 disks.

Guy

-----Original Message-----
From: H. Peter Anvin [mailto:hpa@xxxxxxxxx] 
Sent: Friday, October 29, 2004 4:32 PM
To: Guy
Cc: 'Jim Paris'; 'Neil Brown'; linux-raid@xxxxxxxxxxxxxxx
Subject: Re: RAID-6: help wanted

Guy wrote:
> Could you translate this into English?
> P' = P + D_n + D_n'
> Q' = Q + g^n * D_n + g^n * D_n'
> 
> I understand this much:
> P    =old parity
> D_n  =old data block(s)
> P'   =new parity
> D_n' =new data block(s)
> 
> But is "+" = xor?
> 
> I am lost on this one:
> Q' = Q + g^n * D_n + g^n * D_n'
> 
> With the parity (xor) it can be done by the bit, so an example is easy.
> Can Q be done by the bit, and if so, could you give an example?
> 
> If it takes more than 10 minutes, just tell me it is magic! :)

See my paper on the subject:

http://www.kernel.org/pub/linux/kernel/people/hpa/raid6.pdf

> Also, on a related subject....
> I have a 14 disk RAID5 with 1 spare.
> Once RAID6 seems safe and stable I had hoped to convert to a 15 disk
RAID6.
> Is 15 disks too much for RAID6?
> Any idea what a reasonable limit would be?
> 

The limit is 27 disks total, it's imposed by the md system rather than
RAID-6; 
RAID-6's inherent limit is 254+2 (see, again, the paper.)

        -hpa

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