Re: Re-adding disks to RAID6 in a Fujitsu NAS: old mdadm?

[NeilBrown <neilb@suse.de>]

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On Thu, 28 Jun 2012 11:14:25 +0200 "Stefan G. Weichinger" <lists@xunil.at>
wrote:

> Am 28.06.2012 10:59, schrieb Stefan G. Weichinger:
> 
> >> If you have a machine with the same arch at the NAS, you could
> >>
> >> git clone git://neil.brown.name/mdadm -b mdadm-2.6.5 cd mdadm make
> >> mdadm.static CWFLAGS=-Wall
> >>
> >> and then use the "mdadm.static" on the NAS.
> > 
> > Ok, thanks. I could boot some ubuntu on my Atom-netbook .. or what?
> > Would it be enough to match the 64bit-environment, or would I have to
> > use something with the same kernel ... ?
> 
> Update:
> 
> compiled mdadm.static on a 32bit machine and could re-add the two
> partitions to the array! cool! thanks, Neil!
> 
> ~500 mins to wait now.
> 
> Do I have to fear read-errors as with RAID5 now?

If you get a read error, then that block in the new devices cannot be
recovered, so the recovery will abort.  But you have nothing to fear except
fear itself :-)

> 
> I still don't fully understand if there are also 2 bits of
> parity-informations available in a degraded RAID6 array on 2 disks only.

In a 4-drive RAID6 like yours, each stripe contains 2 data blocks and 2
parity blocks (Called 'P' and 'Q').
When two devices are failed/missing, some stripes will have 2 data blocks and
no parity, some will have both parity blocks and no data (but can of course
compute the data blocks from the parity blocks). Some will have one of each.

Does that answer the question?

NeilBrown


> 
> Thanks again so far, hoping the best now ;-)
> 
> Stefan
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