"Nagios checks to let us know when the filesystem becomes un-writeable" Any specific reason you don't just panic? I've had RO fs edge cases bite me a few times.

"In the future we may even run fiber to these machines" Anything against 10g copper? Per port prices are getting crazy cheap.

"DNS servers, LDAP servers, and our Hadoop Namenodes ... need RAID for extra data safety" Anything you've had issues with? Host level redundancy with the first two has been pretty great IME, can't comment on name odes. The only issue I've run in to is poor LDAP clients not fast failing an unhealthy server when another is available.