Really, you're better off not using any block-level mirroring at all! At scale it makes much more sense to store each (2^24-27) chunk of data on at least 3 inde

Really, you're better off not using any block-level mirroring at all! At scale it makes much more sense to store each (2^24-27) chunk of data on at least 3 independent servers in each facility that has a copy of the dataset (see GFS).