What is VSAM?
- VSAM is Virtual Storage Access Method
- It is a method used to move data between Disk and Main Storage
- VSAM operates in Virtual Environment
- VSAM acts as interface between Operating System and Application Program
File access methods
Data (Records) is retrieved
- Sequential (Reading from beginning to end)
- Random (Records are read by the value in the key)
- Direct (Records are read based on their physical location/address on disk)
VSAM provides all these methods
One access method supporting all types of data retrieval
VSAM Catalogs
All VSAM objects must be cataloged.
- Unlike non-vsam datasets which can be stored in the DASD without a catalog entry and can be accessed by specifying the VOLUME in which they reside
All access on a VSAM object is via the catalog.
- So, it provides means to implement good control over all VSAM objects.
Catalog contains all information about the dataset
- like space, security, access criteria, attributes, processing statistics.
Non-vsam objects can also be defined within VSAM catalog (Including GDGs)
VSAM objects are portable from one system to another.
There are two types of catalog
- VSAM catalog
- ICF catalog
Under ICF
- Each VSAM dataset has VTOC entry
- Space managed by OS space manager
- One primary and 122 secondary allocation