dm-raid vs md-raid

I apologise for being thick, but I trying to understand the difference 
between dm-raid and md-raid. Do these projects overlap, or do they 
address separate problems in the same area?

Some of the information I've looked at suggests they implement similar 
RAID functionality in different ways, while others seem to suggest that 
dm-raid supports RAID functionality provided by device manufactures 
while md-raid implements the RAID functionality internally. I'm getting 
really confused, and can't find anything that explains the difference. 
 From what I've read about dm-raid the concept of maintaining a log of 
changes and being able to catch up the changes rather than having to do 
a full disk rebuild sounds very attractive, and I've also seen it 
mentioned in relation to Clustered LVM which again is attractive.

The architecture I'm trying to build looks something like (improvements 
gladly accepted):

Clustered File System (GFS, OCFS2)
               |
Clustered LVM
               |
Clustered RAID (dm-raid ?)
               |
Networked Disk (GNBD, NBD, AoE)

The aim being to build a fault tolerant, clustered file system where I 
can add a machine to increase file system performance, I can add disks 
to increase storage space, and the whole system can continue to operate 
if a disk or machine fails (assuming you've correctly configured your RAID).

Thanks,

Tony Wright