Dataset Concatenation



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Dataset Concatenation

Postby fornanthakumar » Tue Jan 18, 2011 7:53 am

Hi,

Why should we mention the dataset which has largest block size as first? Any reason...
Regards,
Nanthu.Y.
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Re: Dataset Concatenation

 

Re: Dataset Concatenation

Postby steve-myers » Tue Jan 18, 2011 8:04 am

fornanthakumar wrote:...Why should we mention the dataset which has largest block size as first? Any reason...
This was an issue that was lifted long ago. The obvious reason was the BLKSIZE for the first dataset was used for the entire concatenation.
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Re: Dataset Concatenation

Postby stevexff » Tue Jan 18, 2011 12:46 pm

Yes. I think it was because the OS (originally) only allocated the buffers once, and it used the block size of the first dataset to determine the size. But it doesn't matter these days.
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Re: Dataset Concatenation

Postby Robert Sample » Tue Jan 18, 2011 4:56 pm

From the JCL Language Reference manual:
12.1.6.5 Block Sizes for Concatenated Data Sets


Concatenated data sets can have different block sizes. In a few cases, the data set with the largest block size must appear first in the concatenation. (Note that you can state a value equal to the largest block size for BLKSIZE on the first DD statement, regardless of what the actual block size of this data set is.) Certain data sets can be concatenated in any order of block size; these are:


Partitioned data sets (PDSs), and partitioned data sets extended (PDSEs) without member names coded on the DD statements.

Sequential data sets that are DASD-resident, tape-resident, or in-stream, and are accessed by QSAM and use system-created buffers.

Sequential data sets that are DASD-resident or in-stream, and are accessed by BSAM.

For these data sets, the BLKSIZE obtained is the largest in the concatenation. Note that this block size can cause invalid attribute combinations when combined with the attributes obtained from the first data set in the concatenation.

If you do not specify a block size, the system can, under certain conditions, determine an optimum block size. For detailed information about system-determined block size, see z/OS DFSMS Using Data Sets.
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Re: Dataset Concatenation

Postby fornanthakumar » Wed Jan 26, 2011 5:32 am

Thanks everyone.
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Nanthu.Y.
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