From the author again:
Quote:
For raid6 it is also safe to not sync first, though with the same caveat as raid1. Raid6 always updates parity by reading all blocks in the stripe that aren't known and calculating P and Q. So the first write to a stripe will make P and Q correct for that stripe. This is current behaviour. I don't think I can guarantee it will never changed.


I suppose 'that aren't known' matters. Will you often be writing data that you've just read (so blocks are in cache) or writing enough that you know all the blocks anyway.

Also see: http://www.kernel.org/pub/linux/kernel/people/hpa/raid6.pdf
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LittleBlueThing Running twin 30's