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What is EXCP in VSAM

 
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muthuvel

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Joined: 29 Nov 2005
Posts: 217
Location: Chennai

PostPosted: Wed Mar 01, 2006 2:57 pm    Post subject: What is EXCP in VSAM
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What is an EXCP?in what way it is related to the performance of a VSAM file?
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DavidatK

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Joined: 22 Nov 2005
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Location: Troy, Michigan USA

PostPosted: Wed Mar 01, 2006 8:46 pm    Post subject: Re: EXCP in VSAM
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muthuvel,

EXCP is 'EXecute Channel Program' and gives an indication of the number of I/Os that are happening.

The lower the EXCP count, the lower the number of I/Os and the faster the program will run. For the same amount of data being processed a lower EXCP count means that larger buffers arer being read/written, and for VSAM, maybe the fewer times the program is having to go to DASD to access indexes, The more index space the fewer I/Os.

Dave
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muthuvel

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Location: Chennai

PostPosted: Thu Mar 02, 2006 3:07 pm    Post subject: Re: What is EXCP in VSAM
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Hi,

Thanks for the information.

And may i know how to write an effective code so that the EXCP's are minimal in an VSAM operation.
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DavidatK

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Location: Troy, Michigan USA

PostPosted: Fri Mar 03, 2006 1:10 am    Post subject: Re: What is EXCP in VSAM
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There are a couple of programming considerations when accessing VSAM files depending upon how large the VSAM file is and how many accesses are anticipated.

First, you should random access the VSAM file in Key Order, if possible. This means that you should sort, say, an input file in the Key Order of the VSAM file. This means, that when you read a record from the VSAM file, chances are better that the Key you?re accessing will still be in the index stored in memory.

Second, if you?re accessing a large % of the VSAM file, you may find it more effective to sequentially access the file.

This is about the only things I can think of right off that you can do inside the program.

There are a couple of things you can do about the environment your running in.

In the JCL, you can use:

Code:

//FDNAME DD  SUBSYS=(BLSR,'DDNAME=BLSRXX','MSG=I') 
//BLSRXX DD  DSN=YOUR.VSAM.FILE,                 
//             DISP=SHR                                 


Using the BLSR subsystem, effectively copies the entire VSAM file into memory, so that after the initial load, there will be minimal I/Os. This should only be used when you anticipate a large number of accesses.

Also having your Systems Programmer do some tuning will also help.


Dave
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