HDF5 is a very nice format indeed, and in fact, BLZ is borrowing a lot of good ideas from it. However, HDF5 has its own drawbacks, like not being able to compress variable length datasets, the lack of a query/computational kernel or its flaky resiliency during updates. Also, its approach for distributing data among nodes diverges from our goals.

Finally, you are right, speed is pretty important for us, and we think that our approach can make a better use of current computer architectures.