Can you please provide a new file compatibility option in the H5Pset_libver_bounds function? Currently the only available options for the "low" argument are "earliest" and "latest". It would be good to have a middle alternative requesting the superblock version and internal object versions to match the current 1.8.19 library, or more generally to match the 1.8 series around the time of introduction of HDF5 1.10.
I have several reasons for asking. One is to try to get a good format match with large numbers of netcdf-4 files that my agency previously published using the HDF5 1.8 series. Also I think that migration from 1.8 to 1.10 will be more generally acceptable if 1.10 includes a 1.8 format matching option.
Also, I am suspicious that files created with low = "earliest" may have reduced read back performance in some cases. I do not have current examples or enough information about effects of the different internal object versions to be able to assess this properly. However, performance issues were hinted in the recent discussion about many attributes and many groups.