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alecbcs merged 3 commits into from
Jul 27, 2026
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config: update hdf5/mpich concretization example #576

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config: update hdf5/mpich concretization example
cmelone Jul 27, 2026
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cmelone Jul 27, 2026
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cmelone Jul 27, 2026
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22 changes: 7 additions & 15 deletions tutorial_configuration.rst
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Expand Up @@ -541,25 +541,17 @@ While we're editing the ``spack.yaml`` file, make sure to configure HDF5 to be a
.. 3.externals.out has mpich
.. The concretization result is strange and enables some qt stuff that makes it huge

If you run this as-is, you'll notice Spack still hasn't built ``hdf5`` with our external ``mpich``.
The concretizer has instead turned off ``mpi`` support in ``hdf5``.
To debug this, we will force Spack to use ``hdf5+mpi``.
Now, ``spack spec hdf5+mpi`` will show a dependency on the external MPICH spec.

.. code-block:: spec
To demonstrate how constraints defined in configurations protect against unintended installations, we'll attempt to concretize HDF5 with OpenMPI as the MPI provider.

$ spack spec hdf5+mpi
==> Error: failed to concretize `hdf5+mpi` for the following reasons:
1. 'hdf5' requires conflicting variant values '~mpi' and '+mpi'
2. 'hdf5' requires conflicting variant values '~mpi' and '+mpi'
required because hdf5+mpi requested explicitly
required because ~mpi is a requirement for package hdf5
required because hdf5+mpi requested explicitly
.. code-block:: spec

$ spack spec hdf5+mpi ^openmpi
==> Error: failed to concretize `hdf5+mpi ^openmpi` for the following reasons:
1. cannot satisfy a requirement for package 'mpi'.

In this case, we cannot use the external mpich.
The version is incompatible with ``hdf5``.
At this point, the best option is to give up and let Spack build ``mpi`` for us.
The alternative is to try to find a version of ``hdf5`` which doesn't have this conflict.
Because we require the MPI provider in this environment to be MPICH, Spack will fail to concretize if we explicitly request the use of a different provider.

By configuring most of our package preferences in ``packages.yaml``, we can cut down on the amount of work we need to do when specifying a spec on the command line.
In addition to compiler and variant preferences, we can specify version preferences as well.
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