> Maybe it's possible to have multiple _views_ of a filesystem, so via one API you "see" a flat namespace with hashed directories, and with another a conventional hierarchical one

Spack supports the notion of arbitrary "views" of the store, which can be defined declaratively [1]. Apparently we need to write more highlight blog posts and submit them here b/c people don't seem to know about this stuff :)

For example, if you want to make a view that included only things that depend on MPI (openmpi, mpich, etc.), but not things built with the PGI compiler, in directories by name, version, and what compiler was used to build, you could write:

    view:
      mpis:
        root: /path/to/view
        select: [^mpi]
        exclude: ['%pgi@18.5']
        projections:
          all: '{name}/{version}-{compiler.name}'
        link_type: symlink

That will get you symlinks to the prefixes of the packages that match the criteria, with fancy names.

If you wanted a different layout you might do:

    view:
      myview:
        projections:
          zlib: "{name}-{version}"
          ^mpi: "{name}-{version}/{^mpi.name}-{^mpi.version}-{compiler.name}-{compiler.version}"
          all: "{name}-{version}/{compiler.name}-{compiler.version}"

That puts all your zlib installs in a short name-version directory, most things in directories named by the package and version and a subdirectory for what compiler it was built with, and for packages that depend on MPI it also adds the MPI implementation (openmpi, mpich, etc.) and its version to that.

You can choose to map everything into a single prefix too:

    view:
      default:
        root: ~/myprefix

That is useful for what we call a "unified" environment where there is only one version of any particular package -- it basically converts a subset of a store into an FHS layout.

There are options for choosing what types of links to use and what types of dependencies to link in, e.g., you could exclude build dependencies if you didn't want the cmake you built with to be linked in.

[1] https://spack.readthedocs.io/en/latest/environments.html#fil...