Hi Peter,

Yes, 251 disks for 6 parity.

To build a NxM Cauchy matrix you need to pick N+M distinct values
in the GF(2^8) and we have only 2^8 == 256 available.
This means that for every row we add for an extra parity level, we
have to remove one of the disk columns.

Note that in true, I use an Extended Cauchy matrix that gives the
first row of 1 for "free". This results in N+M <= 256+1.
So, DISKS = 257 - PARITY -> 251 = 257 - 6

A brief introduction of Cauchy and Extended Cauchy matrix can be found in:

Vinocha, "On Generator Cauchy Matrices of GDRS/GTRS Codes", 2012
http://www.m-hikari.com/ijcms/ijcms-2012/45-48-2012/brarIJCMS45-48-2012.pdf
(just check the Introduction, the rest is not related)

More details can be found in:

Roth, "Introduction to Coding Theory", 2006
http://carlossicoli.free.fr/R/Roth_R.-Introduction_to_coding_theory-Cambridge_University_Press%282006%29.pdf
(search for "Extended Cauchy")

Ciao,
Andrea

On Tue, Nov 19, 2013 at 12:25 AM, H. Peter Anvin [off-list ref] wrote:

On 11/18/2013 02:35 PM, Andrea Mazzoleni wrote:

quoted

Hi Peter,

The Cauchy matrix has the mathematical property to always have itself
and all submatrices not singular. So, we are sure that we can always
solve the equations to recover the data disks.

Besides the mathematical proof, I've also inverted all the
377,342,351,231 possible submatrices for up to 6 parities and 251 data
disks, and got an experimental confirmation of this.

Nice.

quoted

The only limit is coming from the GF(2^8). You have a maximum number
of disk = 2^8 + 1 - number_of_parities. For example, with 6 parities,
you can have no more of 251 data disks. Over this limit it's not
possible to build a Cauchy matrix.

251?  Not 255?

quoted

Note that instead with a Vandermonde matrix you don't have the
guarantee to always have all the submatrices not singular. This is the
reason because using power coefficients, before or late, it happens to
have unsolvable equations.

You can find the code that generate the Cauchy matrix with some
explanation in the comments at (see the set_cauchy() function) :

http://sourceforge.net/p/snapraid/code/ci/master/tree/mktables.c

OK, need to read up on the theoretical aspects of this, but it sounds
promising.

        -hpa