I believe you mean you'd like to use mdadm to join storage across multiple physical chassis together? That's already possible, though a bit clunky and fragile compared to some alternatives.
You an use the 'Network Block Device' (NBD) tools to make some devices in another chassis appear on the local chassis, as one option.
Perhaps a better option is to build a per-chassis raid array, and use iSCSI or similair protocols as the transportation layer between chassis' before merging them together for final use.
However, if you're just looking to make a local pair of matching chassis link together in a high-availability/redundant set, per-chassis RAID w/ DRBD linking the two is most likely what you're looking for. DRBD now supports having a third 'not quite up to date' server as a form of off-site disaster-recovery backup server as well.
- WolfWings
