VSAM file reads in CICS online program



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VSAM file reads in CICS online program

Postby MFUser1 » Tue Jul 16, 2013 12:10 pm

Hi, I have a CICS online program which reads a VSAM file only once to fetch a record using key field. Now, the requirement is such that I am changing the program to read the VSAM file 6 times using a different key field each time. The total number of records in the VSAM file at any time will be less than 20.

Does it have any significant impact on the efficiency or response time of my CICS program/transaction because I am changing one VSAM read to 6 reads now ?
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Re: VSAM file reads in CICS online program

 

Re: VSAM file reads in CICS online program

Postby mongan » Tue Jul 16, 2013 12:25 pm

Of course it does. Depending on your program you will probably need 6 times as much time to complete - I/O is very slow compared to the processing time so any increase in I/O will greatly increase your run time.
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Re: VSAM file reads in CICS online program

Postby MFUser1 » Tue Jul 16, 2013 12:37 pm

I read about CICS maintained shared data tables. It says if a file is defined as a CICS-maintained data table (CMT), the source data set and the data table are treated by CICS as a single entity. And data is accessed from memory instead of from DASD. Can we define our VSAM data set as a CICS-maintained data table and exploit this feature so that multiple reads doesn't impact the transaction response times ?
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Re: VSAM file reads in CICS online program

Postby Robert Sample » Tue Jul 16, 2013 4:33 pm

Before you completely redesign everything, why don't you make the change and test it? While I/O is slow relative to processor time (milliseconds versus nanoseconds), the increased I/O may not add that much time to your transaction (depending upon what else the transaction does) -- 5 additional I/O may add as little as a few tenths of a second to the transaction elapsed time. These days, the time to worry about performance is AFTER a performance problem is detected. Making changes to reduce transaction time, without knowing the baseline time for the transaction, is worse than useless -- your application redesign may run slower than the baseline transaction.
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Re: VSAM file reads in CICS online program

Postby MFUser1 » Wed Jul 17, 2013 5:10 am

The baseline time for the transaction is 50ms.
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Re: VSAM file reads in CICS online program

Postby MFUser1 » Wed Jul 17, 2013 5:26 am

Does having a CICS-maintained data table make the use the VSAM file faster ?
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Re: VSAM file reads in CICS online program

Postby MFUser1 » Wed Jul 17, 2013 6:23 am

Hello Robert, sorry for incorrect information earlier, The baseline time for transaction is 200ms.
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