Don't forget that Gluster just released a beta open source Hadoop connector. Their "we'll just a dip a toe in the hadoop community" approach doesn't inspire confidence. On the other hand, they have a decent track record regarding larger HA file system setups and offer many things that MapR offers (e.g. NFS, built as a distributed file system since day 1, etc.) along with open source.
I agree with Joe that there aren't great options with regards to HA. 1. If you're okay with closed source and a big price tag (list is like 4k/node), MapR is probably your best option. 2. If you're Facebook or Yahoo you can make a solution work because you have the manpower. 3. If you're not either 1 or 2, you're kinda stuck on the Hadoop side of things--you use the best hardware you can for the namenode and use either DRBD or a redundant SAN (which can be had for much less than 250k) I strongly believe that things have the potential to change substantially within the next 12 months. (More optimistic than Ryan-- maybe because he has seen the Hadoop community thrashing for longer). And yes, ceph is getting closer all the time.
