Flip The Parity Bits
A five-disk RAID-4 stripe holds four data blocks and one parity block. Look at one corresponding byte from each:
| disk | byte |
|---|---|
| C0 | 10110110 |
| C1 | 01101001 |
| C2 | 11000011 |
| C3 | 00011110 |
| P | XOR of C0..C3 |
A client overwrites C2 with 10101010. The controller uses the subtractive method.
What is the new parity byte, and how many physical disk I/Os does the update cost?
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