Generation(0) refers to the current GDG generation, Generation(-1) refers to the GDG generation just prior to the current generation, and Generation(+1) refers to the GDG generation following the current generation.
When a GDG exists in 3 generations, then the 3rd generation is the current generation, and therefore, we refer to it using generation(0) in JCL.
The generation (2nd generation) right before the current generation (3rd generation) is referenced with generation(-1) in JCL.
Lastly, the generation (1st generation) just before the 2nd generation is referenced using generation(-2) in JCL.
Conclusion
In the 1970s, VSAM was introduced to replace the older IBM file-access methods with greater functionality, and ease-of-use, and to reduce device-dependency issues. The organized index in VSAM makes it easy to access data, and it also supports many datasets within one cluster. For those preparing for an interview in this niche, the above-listed VSAM interview questions will be beneficial to both experienced and new candidates.
