I'd just like to take this opportunity to note that the "md" driver and utilities are absolutely wonderful, and make my life as a sysadmin vastly easier. I use them in strong preference to hardware RAID unless I really need a battery-backup unit so I can enable write caching.
I recently hot-added two more 1TB disks to an existing 8-disk RAID 6 array, and effortlessly reshaped it onto the new disks then grew it to size. Absolutely fuss free and trivial with mdadm. I'm beyond impressed.
Anything that brings this kind of power, control, and reliability to the dmraid layer can only be a good thing.
If the "md" driver knew how to use some battery-backed RAM on a PCIe card as write cache and write-intent bitmap storage, I'd never touch a hardware RAID controller again. As it is, I only do so when I have to run a high performance database, which just *can't* really be done without hardware write caching.
Now, if only LVM as an overlay was md-aware for striping and (sigh) honoured write barriers, so RAID-aware file systems could operate reasonably safely and efficiently on top of LVM-on-MD. It's a shame to have MD's usefulness somewhat crippled by LVM's deficiencies.