Introduction to Files in Mainframe
If you’re coming from a PC or Unix background, you’re used to working with "files." But in the mainframe world, things are a bit different. This handout explains what a data set is, how it differs from regular files, and why understanding this difference is important when working with z/OS.
What Is a Data Set?
- A data set is the mainframe equivalent of a file—but structured to hold records.
- Records are fixed or variable-length chunks of data, used directly by programs.
- Some datasets store text (e.g., code), others store binary (e.g., executables or load modules).
Key Differences
| Concept | PC/Unix File | Mainframe Data Set (z/OS) |
|---|
| Basic unit | File | Data Set |
| Structure | Byte stream | Record-based |
| File types | .txt, .exe, .csv, etc. | SEQ, PDS, PDSE, VSAM, etc. |
| Storage method | Stored in folders | Managed by z/OS, often cataloged |
| Access method | Byte-based (e.g., stream) | Record-based (fixed or variable) |
Cataloged vs. Non-Cataloged
| Term | What It Means | Why It Matters |
|---|
| Cataloged | z/OS knows where the data set is stored | You can access it using just its name |
| Non-Cataloged | You need volume and device info | Harder to locate manually |
Think of the catalog like a search index on your PC. If it's in the index, the system can find it fast.
Common Misconceptions
| Misconception | Reality |
|---|
| “All files are readable” | Load modules (executables) aren’t meant to be opened |
| “Data sets are folders like on Windows” | Only PDS/PDSE have members; other datasets don't |
| “Catalog = Folder” | Catalog = Index; it helps find the data, not hold it |
| “You can read records like lines in Notepad” | Only if it's a text-based dataset with readable encoding |