Btrfs: optionally avoid reads from device replace source drive
It is desirable to be able to configure the device replace procedure to avoid reading the source drive (the one to be copied) whenever possible. This is useful when the number of read errors on this disk is high, because it would delay the copy procedure alot. Therefore there is an option to avoid reading from the source disk unless the repair procedure really needs to access it. The regular read req asks for mapping the block with mirror_num == 0, in this case the source disk is avoided whenever possible. The repair code selects the mirror_num explicitly (mirror_num != 0), this case is not changed by this commit. Signed-off-by: NStefan Behrens <sbehrens@giantdisaster.de> Signed-off-by: NChris Mason <chris.mason@fusionio.com>
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