In this book, we clearly define hierarchical file system problems; expose the minimal set of services that file systems should provide to users to meet their needs. Identifying the problems and services are the first important step to identify solutions for hierarchical file systems problems. We find that the identified problems and the required services are linked, so based on them we propose effective criteria to assess the efficacy of any models for metadata file systems. Also, this research investigates the different dimensions and associated choices that participate in the proposal of new approaches and that affect their ability to improve on current systems. The Cartesian product of those dimensions and options forms a large design space. We prune these dimensions into key items based on criteria that are clearly described to be the first guidance to researchers and the designers in this field. We come up with 48 different models (tagging and at- tributed models). We select 10 models from this set to be described in detail. We formally describe and evaluate these models.