I'm looking for a distributed, fault-tolerant network storage system which exposes block devices (not filesystems) on the clients.

• A client's block device should write simultaneously to several storage nodes
• A client's block device should not fail as long as not all storage nodes backing it went down
• The master should automatically redistribute storages' data when a storage node fails or gets added/ removed
• A single master (which is for metadata only) is fine

So ideally the architecture would be very similar to moosefs (http://www.moosefs.org/) but instead of exposing a real filesystem mounted using a fuse client it'd expose block devices on the clients.

I know of iscsi and drbd but both don't seem to offer what I'm looking for. Or am I missing something?