Hi,
I'm learning QSAM and I found one strange(to me) behaviour of BLKSIZE
parameter.
It's a very simple program:
.............
OPEN (INVMAST,INPUT)
GET INVMAST
CLO CLOSE (INVMAST)
.............
INVMAST DCB DDNAME=INVMAST,DSORG=PS,MACRF=GL,EODAD=CLO,BLKSIZE=32750
............
JCL to run it:
....
INVMAST DD DISP=SHR,DSN=DMAP001.FILE.INVMAST
....
The input data set contains four records and its attribute is: PS, FB,
LRECL=50, BLKSIZE=32700.
It's SMS-managed, too.
Here I want to override BLKSIZE in DCB macro and see what will happen. (Why
I do this? Because I don't fully understand the BLKSIZE parm so I want to
test it. )
I tried many values from 200 to 32750 and the program just runs well.
However, when I decrease the BLKSIZE to 150, I got a S001 abend. The Fault
Analyzer gives me the following description:
A system abend 001 occurred in module IGC0005E at offset
X'1366'.
The abend was caused by machine instruction 05EF (BRANCH AND LINK) in module
TEST CSECT TEST at offset X'34'.
Why 200 works and 150 can not? I notice that the input file contains four
records (4*50 =200) so I add an additional one to it. This time 200 also
gives me a S001 abend! And after i increase the BLKSIZE to 250, it runs
normally.
(The program is tested on z/os 1.7)
What I really don't understand is the relationship of BLKSIZE parm and the
physical structure of the data set. In ISPF DSLIST I can see a data set has
BLKSIZE=32720. Does this mean the physical blocksize of the data set is
32720? If it's true, when reading it, why we're allowed to spefify BLKSIZE
parm in DCB macro?
i also searched the archive and found out that for PDSE it's physical
bolocksize is always 4K and the BLKSIZE parm we specify is called 'logical
blocksize' - it only has impact on buffer size of access method. How about
other data sets?
Regards,
Johnny
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