[h5md-user] Dataset layouts

[Peter Colberg]

Hi all,

While working on a truly large-scale scientific application using
the HDF5 MPI I/O driver, I noticed it would be useful to mandate that
“meta-datasets” are always stored with compact layout. This would
apply to proposal 100 (scalar step/time), and `box/edges` for the
fixed-geometry case.

When using parallel HDF5, most C API calls are collective, which means
that a function must be called by all processes in the communicator
used to access a file. Compact datasets are metadata and therefore
must be written/read collectively. This goes nicely with the other
metadata stored in attributes.

In similar spirit, the specification states that time-dependent
datasets have an infinite first dimension. I suggest we specify
explicitly that these datasets must have a chunked layout.

Regards,
Peter