Question

Assume that the 4 data strips (also called data blocks) within a certain stripe on a RAID 5 system are designated B0, B1, B2 and B3. The corresponding parity block P_old for this stripe is computed as the cumulative XOR of the 4 data strips within the stripe:

P_old =B0 ^ B1 ^ B2 ^ B3. Hence each stripe contains a parity block and 4 data blocks.

Block B2 is to be overwritten with the new contents designated as B2_new.

a) Write down an expression for the corresponding new parity block P_new that requires reading no more than two blocks of the 5 blocks in the stripe.

P_new = __________

b) Write down a different alternate expression for the new parity block P_new if there are no restrictions on the number or type of blocks read.

P_new = __________

Answer 1