HFS - Hierarchical File System
HFS (Hierarchical File System) is a file system implementation within z/OS UNIX System Services (USS) that provides a POSIX-compliant, byte-stream oriented, tree-structured directory and file organization. It allows z/OS to support UNIX-style applications and provides a familiar file system structure for developers accustomed to distributed environments.
Key Characteristics
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- POSIX Compliance: Adheres to POSIX standards, supporting features like case-sensitive filenames, file permissions (read, write, execute for owner, group, others), symbolic links, and hard links.
- Byte-Stream Oriented: Files are treated as unstructured streams of bytes, unlike traditional MVS data sets which often have record-oriented structures.
- Tree-Structured Directory: Organizes files and directories in a hierarchical tree structure, starting from a root directory (
/), similar to UNIX or Linux file systems. - Integration with z/OS UNIX System Services (USS): HFS files and directories are accessed and managed primarily through USS commands, shell scripts, and APIs, providing a native UNIX environment on the mainframe.
- Mountable File System: HFS file systems can be mounted and unmounted, allowing for flexible management of storage volumes and resource allocation.
- Security Integration: Access to HFS files and directories is controlled by z/OS security managers like RACF, using UNIX-style permissions combined with traditional mainframe security profiles.
Use Cases
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- Hosting UNIX Applications: Provides the file system foundation for running native UNIX applications, utilities, and tools compiled for z/OS UNIX System Services.
- Storing Shell Scripts and Executables: Used to store shell scripts (e.g.,
bash,ksh), compiled C/C++ programs, Java executables, and other USS-native binaries. - Supporting Java Virtual Machines (JVMs): Java applications and their associated libraries, configuration files, and log files are typically stored and accessed within HFS directories.
- Interfacing with Distributed Systems: Facilitates data exchange with distributed platforms via protocols like FTP, NFS, or SMB, as HFS provides a compatible file system structure.
- Application Development and Testing: Offers a familiar development environment for programmers accustomed to UNIX-like systems, enabling them to develop and test applications directly on z/OS.
Related Concepts
HFS is a foundational component of z/OS UNIX System Services (USS), enabling the mainframe to host a POSIX-compliant environment alongside traditional MVS workloads. It contrasts significantly with traditional MVS data sets (e.g., sequential, partitioned, VSAM) by offering a byte-stream, hierarchical, and case-sensitive file system structure, though both can coexist and be accessed from USS. While HFS was the original file system for USS, it has largely been superseded by zFS (z/OS File System), which offers superior performance, scalability, and recoverability, making HFS a legacy but still supported option. Security for HFS is managed by RACF (or equivalent security managers), integrating UNIX-style