In order to save memory space, the logic for transient data and partitioned data is quite the same thing, that’s why in the documentation for transient data we said that:
The general idea is to take the static formats described above and use them as a base to append all the time dependent data. As such, a file holding static data has a very similar structure to a file holding dynamic data.
So in your case if we take the unstructured grid, the documentation said for partitioned data :
We describe the split into partitions using HDF5 datasets
NumberOfConnectivityIds,NumberOfPointsandNumberOfCells. Let n be the number of partitions which usually correspond to the number of the MPI ranks.NumberOfConnectivityIdshas size n where NumberOfConnectivityIds[i] represents the size of theConnectivityarray for partition i
We can mirroring that here, instead of having n ranks we have here n timesteps and so for unstrutured grid we need to define several array of size n:
- NumberOfConnectivityIds
- NumberOfPoints
- NumberOfCells
However it’s not sufficient that why we also have another group named Steps.
Finally, after writing this post, its clear for me that if someone is only interested on transient data the documentation isn’t really easy to understand, we may want to improve it.
EDIT: I open an issue here https://gitlab.kitware.com/vtk/vtk/-/issues/19242