Just enhance data structures to record a second device per slot to be
used as a 'replacement' device, replacing the original.
We also have a second bio in each slot in each stripe_head. This will
only be used when writing to the array - we need to write to both the
original and the replacement at the same time, so will need two bios.
For now, only try using the replacement drive for aligned-reads.
In this case, we prefer the replacement if it has been recovered far
enough, otherwise use the original.
This includes a small enhancement. Previously we would only do
aligned reads if the target device was fully recovered. Now we also
do them if it has recovered far enough.
Reviewed-by: Dan Williams <dan.j.williams@intel.com>
Signed-off-by: NeilBrown <neilb@suse.de>
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