The description of DRBD as software RAID is a bit misleading, since in most deployment scenarios, only the "primary" node of the pair can actually write data to

The description of DRBD as software RAID is a bit misleading, since in most deployment scenarios, only the "primary" node of the pair can actually write data to the replicated volume. It's real use is in building high-availability services -- since the "mirrored" disks aren't in the same box, you can have an up-to-the-second copy of the data there. The two systems don't even have to be in the rack next to each other, which is a tough trick to pull off with normal hardware RAID. As I said in my other comment, though, it is a lot slower than native disk writes.