Esperimenti con RAID6 (2026)

Il codice python che segue consente di provare sperimentalemnte le metodologie di fault tolerance descritte nell'articolo "EVENODD: An Optimal Scheme for Tolerating Double Disk Failures" di Mario Blaurn et al.

#!/usr/bin/env python3

def print_table(tab, nrow, ncol):
	for i in range(nrow):
		for j in range(ncol):
			print(tab[i][j], end=" ")
		print()
	print()
	
# delete a column
def fail(tab, nrow, f):
	for i in range(nrow):
		tab[i][f] = 'x'

# compute row parity
def compute_parity(tab, nrow, ncol):
	for i in range(nrow):
		p = 0
		for j in range(ncol):
			p ^= tab[i][j]
		tab[i][ncol] = p

# compute diagonal parity
def compute_dparity(tab, nrow, ncol):
	p = [0] * ncol
	for i in range(nrow):
		for j in range(ncol):
			p[(i+j) % ncol] ^= tab[i][j]
#	print(p): the last parity is encoded
# if s negate all the diag parity values
	s = p[ncol - 1]
	for i in range(nrow):
		tab[i][ncol + 1] = p[i] ^ s

# restore row parity
def restorep(tab, nrow, ncol, f):
	for i in range(nrow):
		p = tab[i][ncol]
		for j in range(ncol):
			if j != f:
				p ^= tab[i][j]
		tab[i][f] = p

# restore diagonal parity
def restored(tab, nrow, ncol, f):
	p = [tab[i][ncol + 1] for i in range(nrow)] + [0]
	for i in range(nrow):
		for j in range(ncol):
			if j != f:
				p[(i+j) % ncol] ^= tab[i][j]
# the last parity is encoded
# if s negate all the diag parity values
	s = p[(f - 1) % ncol]
#	print(p, s)
	for i in range(nrow):
		tab[i][f] = s ^ p[(f + i) % ncol]

# restore 2 data columns
def restore2(tab, nrow, ncol, f1, f2):
	s = 0
	for i in range(nrow):
		s ^= tab[i][ncol] ^ tab[i][ncol + 1]
	p = [s ^ tab[i][ncol + 1] for i in range(nrow)] + [s]
#	print(s, p)
	nindex = (nrow + f2) % ncol
	for _ in range(nrow):
		v = p[nindex]
#		print(nindex, (nindex + 1) % ncol)
		for i in range(nrow):
			j = (nindex - i) % ncol
			if j != f1:
				v ^= tab[i][j]
			else:
				row = i
#		print(row)
		tab[row][f1] = v
		nindex = (row + f2) % ncol
		v = tab[row][ncol]
		for j in range(ncol):
			if j != f2:
				v ^= tab[row][j]
			tab[row][f2] = v

# MAIN
def main():
# input the table
	tab=[]
	while True:
		line = input("")
		if len(line) <= 1:
			break
		tab.append([int(x) for x in line.split()] + [0, 0])
	nrow = len(tab)
	ncol = len(tab[0]) - 2
# add parities row + diag
	compute_parity(tab, nrow, ncol)
	compute_dparity(tab, nrow, ncol)
	print_table(tab, nrow, ncol + 2)

	savetab = tab

	while True:
		line = input("failure ")
		s = line.split()
# failures: 1 or 2 columns
		if len(s) == 2:
			f1, f2 = int(s[0]), int(s[1])
		elif len(s) == 1:
			f1 = f2 = int(s[0])
		else:
			return
	
		fail(tab, nrow, f1)
		fail(tab, nrow, f2)
		print_table(tab, nrow, ncol + 2)
		if f1 == f2:
			print("1 faulty", f1)
# missing row parity
			if f1 == ncol:
				compute_parity(tab, nrow, ncol)
# missing diag parity
			elif (f1 == ncol + 1):
				compute_dparity(tab, nrow, ncol)
# missing data 
			else:
				restorep(tab, nrow, ncol, f1)
		else:
			if f1 > f2:
				f1, f2 = f2, f1
			print("2 faulty", f1, f2)
			if f1 == ncol:
# missind both parities
				compute_parity(tab, nrow, ncol)
				compute_dparity(tab, nrow, ncol)
# missind data + row parity
			elif f2 == ncol + 1:
				restorep(tab, nrow, ncol, f1)
				compute_dparity(tab, nrow, ncol)
# missind data + diag parity
			elif f2 == ncol:
				restored(tab, nrow, ncol, f1)
				compute_parity(tab, nrow, ncol)
				pass
# missing two data columns
			else:
				restore2(tab, nrow, ncol, f1, f2)
		print_table(tab, nrow, ncol + 2)
		if tab == savetab:
			print('OK')
		else:
			print('ERR!!!!')

if __name__ == "__main__":
	main()