Re: Rewrite md raid1 member

[NeilBrown <nfbrown@novell.com>]

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On Fri, Aug 19 2016, Chris Dunlop wrote:

>
> In my case, I want it to write everything.
>
> If I do my 'dd' to write everything as previously described, with the window
> of opportunity for stale data to end up on the written disk, one option
> would to run a scrub / repair to check the data is the same - but if I'm
> unlucky with my dd and the data isn't the same for some sector[s], I want to
> ensure the correct data is copied over the stale data and not the other way
> around, e.g. to specify "in the event of a mismatch, use the data from sda
> and overwrite the data on sdb".
>
> Unfortunately I don't know how that can be done.
>
> Does anyone know?

If it is the second device in the array (as listed by mdadm --detail)
then you can stop the array and re-assemble with --update=resync.

If it is the first device I can only suggest that you
fail the device and add it again:

 mdadm /dev/mdXX --fail /dev/sdYY
 mdadm /dev/mdXX --remove /dev/sdYY
 mdadm /dev/mdYY --add /dev/sdYY

If the "good" drive fails during the rewrite it might be a little bit
fiddley getting the array working again, but all the data will certainly
be there on the device you are re-writing, so you won't lose anything.

NeilBrown

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