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What is the significance of blksize

 
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girish.kanswa
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PostPosted: Wed Jul 09, 2008 5:22 pm    Post subject: what is significance of blocksize in JCL ..?
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Hi All,

Can you please tell me what is the significance of blksize in JCL..?

Thanks in advance
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expat

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PostPosted: Wed Jul 09, 2008 5:47 pm    Post subject:
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I generally do not specify it, just let the O/S do the work for you as it will select the best BLKSIZE for the output device being used.
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Anuj Dhawan

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PostPosted: Wed Jul 09, 2008 6:20 pm    Post subject:
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Hi,

If you are searching for the basic (in JCL world) definition then,

The BLKSIZE parameter is an optional parameter of the DD statement which provides the block size, in bytes, to use for the largest block to be processed for the associated data set.

Well, lately this question is been asked in many threads, just a coincidence or..anyways here are the links for some similar threads..

http://www.ibmmainframes.com/viewtopic.php?t=32215&highlight=blksize

http://www.ibmmainframes.com/viewtopic.php?t=32057&highlight=blksize

Please learn to search the forums.
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Bill Dennis

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PostPosted: Wed Jul 09, 2008 6:41 pm    Post subject:
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Combining small records into blocks make I/O and data storage much more efficient and can reduce elapsed time to run a job. A file's BLKSIZE can be specified in a program but the best practice is to allow the JCL or system to set it. Most times you can specify BLKSIZE=0 and the system determines the best blocksize for the device.
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dick scherrer

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PostPosted: Wed Jul 09, 2008 9:58 pm    Post subject:
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Hello,

Quote:
Can you please tell me what is the significance of blksize in JCL..?
The main significance of the block size is the difference between physical and logical input/output actions. Basically, if your data has one record per block, every single read or write requires physical i/o. If your data has 1000 records per block, the number of physical reads or writes for 1000 records is one.

Another very important consideration the block size determines is how efficiently your data is allocated on the media (dasd more so than cart/tape). Efficient block sizes can greatly reduce the amount of media needed to store the data.
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expat

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PostPosted: Thu Jul 10, 2008 12:08 pm    Post subject:
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Quote:
Efficient block sizes can greatly reduce the amount of media needed to store the data.

Yes, there is a gap between every block that is stored.
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Anuj Dhawan

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PostPosted: Thu Jul 10, 2008 12:18 pm    Post subject:
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expat wrote:
Yes, there is a gap between every block that is stored.
And this is more paining when a media can be accessed in linear way only..such as TAPE media.
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