half a PB?
How big is that like 1/4 a rack? 1/2 a rack? not much to write home about..
ddn website says up to 3.4PB/rack.
One thing I learned in the past few months from a former DDN'r is that their stuff is not designed for 24x7 availability, but rather designed so that there can be scheduled downtime for maint. Not an issue for research or edu or HPC which are probably usually project based.. just keep it in mind if your considering DDN as a more general purpose storage system (hopefully your not but their sales force tries to pitch it as such).
It is halfway decent if you have a large amount of storage (10s of PB etc) due to it's lower cost vs traditional tier 1 (or even tier 2) systems.