Replying to Arjan van de Ven:
Because it _is_ brand spanking new (and thus unclean from a RAID point of view)?
but.. it might as well be marked clean. there's no information in never-used parts of the disk after all... so why sync it instead of pretending it's synced perfectly fine....
raid1 means that contents of both mirrors are exactly the same. Just after creation, this is obviously not true. Linux MD implementation solving this problem by synching it.
Other solution may be to store list of written blocks and do something with this, but, I think this isn't reliable/scalable.
Anyway, as I said previously, root volume reconstruction can happily continue after firstboot (and second boot too), but inconsistency in /boot making system unbootable.