On Friday May 5, rowe@xxxxxxxxxxxxx wrote:
>
> > Sorry, I couldn't find a diplomatic way to say you're completely wrong.
>
> We don't necessarily expect a diplomatic way, but a clear and
> intelligent one would be helpful.
>
> In two-disk RAID5 which is it?
>
> 1) The 'parity bit' is the same as the datum.
Yes.
>
> 2) The parity bit is the complement of the datum.
No.
>
> 3) It doesn't work at a bit-wise level.
No.
>
> Many of us feel that RAID5 looks like:
>
> parity = data[0];
> for (i=1; i < ndisks; ++i)
> parity ^= data[i];
Actually in linux/md/raid5 it is more like
parity = 0
for (i=0; i < ndisks; ++i)
parity ^= data[i];
which has exactly the same result.
(well, it should really be ndatadisks, but I think we both knew that
was what you meant).
>
> which implies (1). It could easily be (2) but merely saying "it's not
> data, it's parity" doesn't clarify matters a great deal.
>
> But I'm pleased my question has stirred up such controversy!
A bit of controversy is always a nice way to pass those long winter
nights.... only it isn't winter anywhere at the moment :-)
NeilBrown
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