Thanks for your feedback, it makes sense that lots of small attributes will be slower to read than a big attribute. The whole point of using hdf5 is performance, so your input is very valuable.
In your proposal, we should add extra information in the AMRBox Dataset which correspond to the underlying image data: WholeExtent, Origin, Spacing, Direction and Piece Extent if applicable. In hdf5 it should be attributes under a group.
Hence we may need a GROUP AMRBoxes with several GROUP AMRBox with all the required data as attributes. Unless you think it could also be a bottleneck?
@danlipsa any thought?