Re: RAID6 on PowerPC is faulty

On Thursday September 15, hpa@zytor.com wrote:

If you have been running RAID6 on a PowerPC with Altivec, your RAID is
not two-disk redundant, due to bad Q syndrome computation.

It *is* one-disk redundant, however.

The following patch will fix it:

http://www.kernel.org/pub/linux/kernel/people/hpa/raid6-altivec-fix.patch

.. however, it will not correct an existing array without rewriting
every stripe.

Neil, is there any way to force md to recompute all the syndromes?

mdadm --assemble --update=resync ....

should trigger a resync which should recalculate all P and Q (provided
your array isn't degraded).
This requires shutting the array down of course, but then so would
installing the patch.

I'm planning a feature whereby you can trigger a resync-like pass
which checks all redundancy and either report or reports-and-corrects
errors.  It's still a little way off though.

NeilBrown