About VSAM file.

[rejip]

Hi,

Can anyone help me what is the concept behind VSAM datasets. Why is it called as Virtual Storage Access Method.


Thanks & Regards,
Reji P.

 

[Dimandja]

Hi rejip,

Very good question indeed.

As you know, IBM had been through SAM (Sequential Access Method), ISAM (Indexed...), and I guess combining these database formats with a few new tricks (Relative Data Set, etc...) had IBM looking for a new all encompassing database name.

Virtual sounds pretty good when you think you have it all covered, just like 'virtual memory' came about when those limitations were 'licked'.

Dimandja

 

[3gm]

I suspect IBM chose that because it was the first access method to allow for true virtual storage (Virtual Storage Access Method). In prior access methods, buffers had to be located at fixed, real storage locations within the partition and the access method code generally had to reside within the partition rather than in shared, pageable storage. VSAM eliminated those constraints (and provided a bunch of other benefits as well).

BTW, I'm not sure I buy into calling it a database. It certainly isn't a DBMS. It also provides no tools for updating or querying the data beyond simply record replace and print functionality in the IDCAMS utility. It IS the underlying access method for true DBMS systems like IBM's DL1.

Glenn
Brainbench MVP for COBOL II

 

[Dimandja]

For what it's worth, the latest IBM 'database' DB2 is implemented on top of VSAM.

Dimandja