Hi David, On Tue, Apr 17, 2012 at 10:18:55PM +0200, David Brown wrote: [...] > For quad parity, we can try g3 = 8 as the obvious next choice in the > pattern. Unfortunately, we start hitting conflicts. To recover you should not use 8, because this is not a generator of GF(256) with polynomial 285, the standard for the RAID-5/6 setup. This means than 8^k does not cover the complete field for k in [0 254], thus having cycles and, consequently, creating conflicts. Some generators could be: 2, 4, 6, 9 13, 14, 16... but not 32 nor 64. I know that powers of two are nice, but if you want to have generic RAID, you must use other values. The log/exp tables, are, of course, always valid. BTW, the GF(256) with polynomial 285 has exactly 128 generators, so it would be possible to have up to 129 parity disk (1 is not a generator), for, I guess, a max of 256 disks (or maybe 255?). Hope this helps, bye, -- piergiorgio -- To unsubscribe from this list: send the line "unsubscribe linux-raid" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
