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But then imagine that you have RAID 5. Now you have three drives: two for data and one for parity. Now suppose that you overwrite a sector again. Before writing your disks contain: (A1), (B1) and (A1 XOR B1). You overwrite (B1) with (B2). To do so you write (B2) to the second disk and (A1 XOR B2) to the third. A power failure occurs again… And then, on the next boot, you also find out that disk 1 (one that you didn’t write anything to) is dead. You might think that you can still reconstruct your data because you have RAID 5 and 2 disks out of 3 are still alive.
But imagine that disk 2 succeeded to write new data, while disk 3 failed. Now you have: (lost disk), (B2) and (A1 XOR B1). If you try to reconstruct A from these copies you’ll get (A1 XOR B1 XOR B2) which is obviously not equal to A1. Bang! Your RAID5 has corrupted the data that you didn’t haven’t even write been writing at the time of the power loss.
Because of this problem, Linux `mdadm` refuses at all to start an incomplete array after unclean shutdown. There’s no solution to this problem except full data journaling at the level of each disk drive. And this is… exactly what Ceph does! So, Ceph is actually safer than RAID. :)
