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@zytor.com] 
Sent: Friday, October 29, 2004 4:32 PM
To: Guy
Cc: 'Jim Paris'; 'Neil Brown'; linux-raid@vger.kernel.org
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