ESDS / Non-VSAM Sequential File 1

[chmanio]

Hi Everybody!

Can anybody please answers these 2 questions?

1. What's the difference between VSAM-ESDS and Non-VSAM
sequential file?

2. What will happen to the created AIX if the base cluster
was deleted?

your answer will be greatly appreciated!

thanks!

Chris

 

[3gm]

1. A non-VSAM sequential file is simply a sequential file somewhere; could be disk, tape, cards(!), printer (or spooler) for example. On a z\OS(MVS) system, these files are normally accessed via QSAM IO modules. A VSAM ESDS is a file located on disk in VSAM-managed space (i.e. is listed in the VSAM catalog) and is accessed via VSAM IO modules. As far as COBOL is concerned, either type of file is OK for use as a SEQUENTIAL file with no program changes needed.

2. ESDS files do NOT have an associated AIX as the file is not indexed. If you delete the ESDS cluster, you've deleted the entire file.

Regards.

Glenn

 

[chmanio]

Thanks for replying Glenn!

 

[Truusvlugindewind]

Not sure, but someone told me once that you can update/delete records in a ESDS-VSAM dataset, which is not possible when you process QSAM.
Never tried it though.

 

[WMK]

Little known fact (and even LESS often used),

You actually MAY use the REWRITE statement for a QSAM file on disk (not tape) if OPEN I-O.

See:

http://publibz.boulder.ibm.com/cgi-bin/bookmgr_OS390/BOOKS/igy3pg30/1.9

and

http://publibz.boulder.ibm.com/cgi-bin/bookmgr_OS390/BOOKS/igy3lr30/6.2.26.3

(and/or try it if you don't believe me) <G>

Bill Klein

 

[razalas]

Just to back Bill up (not that he needs any backup),

I have REWRITE'n QSAM records on a sequential file in an IBM MVS (zOS) environment.

One caveat, as noted in Bill's first link, is that you CANNOT do anything to change the record length of the record you are rewriting. This will cause an abend, but I don't remember the abend codes it gives. But as long as you are rewriting a record of the same length, you won't have any problems.

Code what you mean,
and mean what you code!
But by all means post your code!

Razalas

 

[stevexff]

Originally VSAM supported RRDS (BDAM replacement), KSDS (ISAM replacement) and ESDS (Entry Sequenced Data Set), which is VSAM-speak for sequential file. They never really caught on that well, as they didn't offer any advantages over QSAM, and in the early days by the time you had defined the dataspace and the dataset within it they were more trouble than they were worth.

You can update records in place, but you can't delete them, although you can get around this by having a field on the record that identifies it as being logically deleted. Eventually at some stage you are supposed to copy the file and drop all the logically deleted records. But it's not very clean as it requires your application to know what the flag means.

Unless the file already exists and you are effectively stuck with it, you might find it easier to use QSAM...