A hierarchical file system can be in a dataset type called HFS, or it can be in a zFS dataset. HFS datasets can span many dusty the volumes. If spanned, they must be SMS managed. The zFS and HFS are both Unix file systems and both can participate in shared CIP fluxes. However, while HFS always has a single file system per dataset, zFS can have multiple file systems in a single dataset. These datasets are called aggregates and the array collection of datasets. HFS datasets are like PDAC datasets that allocated like any other z/OS dataset. Although an HFS dataset is a z/OS dataset, an MVS open request fails, users must use Unix system services to access data in such a dataset. The hierarchical file system consists of one root file system and multiple user file systems. The root file system is the base for HFS and is at the top of the hierarchy. HFS datasets are allocated at z/OS datasets, so DSN type equals HFS. HFS datasets can only be opened by Unix system service kernel operations, and HFS datasets can reside on the same volumes as other z/OS datasets. Non SMS managed HFS datasets are often used. Now a few other characteristics. The size of the dataset is limited to two gigabytes, if single volume. A multi volume HFS can span up to a maximum of 59 volumes. HFS can have up to a maximum of a 123 extents per volume, and the multi-volume HFS can have up to a maximum of 255 total extents for all volumes. Also, keep in mind that within a file system in HFS, there is no way to control the amount of space each user can use. It's actually possible for one user to use the entire space in a file system, so the recommendation here is that each user have their own file system. A zFS aggregate is a VSAM linear dataset for an LDS that contains one or more zFS file systems. The name of the aggregate is it's VSAM LDS name. A zFS file system is a logical named entity contained within the zFS aggregate that has a root directory and can be mounted. It has a logical maximum size known as it's quota. A zFS file system is a named entity that resides in a zFS aggregate. It contains a root directory and can be mounted into the USS hierarchy. File system is not really a new term, but a zFS file system resides in a zFS aggregate, which is different from an HFS file system. Now, for compatibility mode aggregates, the VSAM LDS name is the same as the zFS aggregate name is the same as the zFS file system name. The culture of these file systems is set to the size of the aggregate. A compatibility mode aggregate can contain only one zFS file system, making this type of aggregate more like an HFS file system. This is flagged in the aggregate when it's created. Version file systems in a shared file system that are no longer being used can be automatically unmounted. Also, note that the multi file system aggregates are no longer supported by z/OS. A colony address space runs USS system code as an extension to the USS kernel address space. zFS runs as a Unix system services colony address space. There must be an entry in a BPXPRMx parmlib member for CVS and zFS proc must be available. The BPXPRMx parm member in the parmlib contains a file sys type statement for the zFS physical file system. It includes an AS name parameter specifying the zFS proc. AS name is required for PFS's that run in colony address spaces. zFS can now handle mounts of zFS file systems contained in the BPXPRMx parm. Now commands can be left in /ETC, /RC, or move into BPXPRMx parm. zFS admin commands, the query information, for example, ls, fs, and ATTR info can be issued by any user. zFS admin commands that modify, for example, zFS admin create require that the user be logged in as root. Now, what are the advantages of zFS? Well, zFS is a file system that is fundamentally different from HFS. The administration view of zFS is different from HFS. zFS file systems are created in a different manner from the way that HFS file systems are created. But in general, however, the application view of zFS is the same as the application view of HFS. The same APIs and commands are used for zFS as are used for HFS. Once a zFS file system is mounted, it's almost indistinguishable from a mounted HFS file system. zFS is a z/OS Unix file system that can be used in addition to the hierarchical file system. zFS file systems contain files and directories that can be accessed with the z/OS hierarchical file system, file application programming interfaces or API's. zFS file systems can be mounted into the z/OS Unix hierarchy along with other local or remote file system types. So for example, HFS, TFS, auto mount NFS and so forth. zFS does not replace HFS, rather it's complimentary to it, and importantly, zFS is the strategic Unix system services file system for z/OS. Now, in the next video, we'll take a closer look at using the HFS and zFS file systems.
