Online filesystem scrubbing and repair

You need more than a flag. You need an index number.
There might be several ways to retrieve a given block. By default, some optimal choice is made.
In "desperate" circumstances, you want to iterate through all possible mechanisms in turn.
For example, if you have RAID6, you can read the block directly, construct it from all other data blocks plus parity, from all other data blocks plus Q, or from parity and Q and all-but-2 of the data blocks. A total of n possible sources if there are n devices in the array.
For an n-drive RAID1, there are obviously n possible sources too.