Here’s another low-level server for both the FS and 5N. This one allows exporting files and block devices as block devices.
NBD is normally used to serve root filesystems for diskless systems, but in the case of the Drobos it can be used for low-level maintenance and recovery, as explained in this thread: http://www.drobospace.com/forums/showthread.php?tid=141627&pid=180086#pid180086
By default, this DroboApp exports /dev/sda2. That is an unused and unformatted partition that exists on all FS and 5Ns. It should be safe to use for testing, although I recommend not using for anything serious.
I do not recommend creating new filesystems over NBD (i.e., use mke2fs). When I tried it the NBD server died on me. I’m not sure if this was a fluke or if it is a sign of a more serious underlying problem.
In any case, if you do want to create an EXT3/EXT4 partition on /dev/sda2 here are the commands that you should use to create a Drobo-compatible filesystem:
mke2fs -j -i 262144 -m 0 -O sparse_super,uninit_bg,^resize_inode /dev/nbd0
mkfs.ext4 -i 262144 -K -m 0 -E lazy_itable_init -O dir_index,sparse_super,uninit_bg,^resize_inode /dev/nbd0
As usual, comments, suggestions, and feedback are welcome.