Hi again David,

Yes, being a generator for GF(2^8) is a requirement for a parity
generator (sorry for the confusing terminology here - if anyone has
a better suggestion, please say) to be part of a 255 data disk
system. However, being a GF generator is necessary but not
sufficient - using parity generators (1, 2, 4, 16) will /not/ give
quad parity for 255 data disks, even though individually each of 1,
2, 4 and 16 are generators for GF.

I ask again, could you please elaborate this?
I nowhere found such a further constrain for the parities.

All I could find is that the Vandermonde matrix must
be done with generators.

255 data disks is the theoretical limit for GF(2⁸).  But it is a
theoretical limit of the algorithms - I don't know whether Linux md
raid actually supports that many disks.  I certainly doubt if it is
useful.

The reason to use many disks is in case of
geo-redundant RAID, for example with iscsi.
In this situation you want to have a lot of
redundance, in parities, not mirror.

bye,

-- 

piergiorgio
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