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Waiting for Volumes

 
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gylbharat

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Joined: 31 Jul 2009
Posts: 565
Location: Bangalore

PostPosted: Fri Aug 19, 2011 12:33 pm    Post subject: Waiting for Volumes
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Hi,

I observed a strange thing. I was running two parallel jobs - Unload from IMAGE COPY for two different tables.

The image copy of both the tables reside on the same tape volume.

When both the jobs were running simultaneously - I saw one job was actually waiting for the Volume to be available.

Both the jobs were using disposition as SHR. but still not sure why the job was waiting for the volume.
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dick scherrer

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Joined: 23 Nov 2006
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PostPosted: Fri Aug 19, 2011 12:44 pm    Post subject:
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Hello,

Why in the world would you use SHR for a tape? It makes no sense (imagine the poor tape drive trying to spin back and forth to take turns) to consider sharing tape. There are some reasons to use shr for a tape, but they have nothing to do with your request.

Two processes cannot allocate/use the same tape.
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vasanthz

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Joined: 28 Aug 2007
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PostPosted: Fri Aug 19, 2011 12:49 pm    Post subject:
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Hello D,
I am aware it is normal that when a job uses a tape, the other jobs which require that tape would wait until the tape is available.
Just curious, Is the same true for VTS tapes as well?
I have heard that one of our clients use VTS tapes for some datasets, but I have not used them so far to test it out.
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dick scherrer

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PostPosted: Fri Aug 19, 2011 1:01 pm    Post subject:
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Hi Vasanth,

Quote:
Just curious, Is the same true for VTS tapes as well?
As far as i know. . .

I've been thru several v-tape migrations, but these systems would not promote a job to producton with SHR specified for a tape unit so i never tried.

When data is migrated to v-tape, there need be no program changes and the only jcl change i remember is to use the new generic/esoteric (several use UNIT=VTAPE).
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gylbharat

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PostPosted: Fri Aug 19, 2011 2:31 pm    Post subject:
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Thanks Dick ... I was not able to figure out why it was actually happening... Thanks....

Can you also please elaborate on - when to use SHR for TAPE ?
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prino

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Joined: 07 Feb 2009
Posts: 984
Location: Oostende, Belgium

PostPosted: Fri Aug 19, 2011 2:57 pm    Post subject:
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gylbharat wrote:
Thanks Dick ... I was not able to figure out why it was actually happening...

And you call yourself a "Software Engineer"?

How the fluffing hell did you ever get your current job, when you cannot even figure out that, as DS wrote, you cannot read a tape at two places at the same time?

Please change the title in your profile to "Mainframe novice", your current title is about 42 levels over your qualifications.
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Robert Sample

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Joined: 06 Jun 2008
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PostPosted: Fri Aug 19, 2011 3:00 pm    Post subject:
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You can use DISP=SHR for a tape data set any time you want. The tape will not be shared, of course, since the tape drive (real OR virtual) can only be at one spot on the tape at a time.

From the JCL Language Reference manual, section 12.19.2.1:
Quote:
If you specify DISP=SHR for an output tape data set and (1) the data set is not protected by RACF or a password or (2) the data set has no expiration date, the system does not verify the data set name in the header label.
So there are reasons other than data set sharing to use DISP=SHR.

And Appendix 1.2.2.3 of the JCL User's Guide help clarify the usage:
Quote:
Indicates that you are willing to share the data set
with other jobs. This subparameter alone, however,
does not guarantee that sharing will take place. See
z/OS DFSMS Using Data Sets for a full description of
data-set sharing.
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gylbharat

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PostPosted: Fri Aug 19, 2011 3:01 pm    Post subject:
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prino wrote:
gylbharat wrote:
Thanks Dick ... I was not able to figure out why it was actually happening...

And you call yourself a "Software Engineer"?

How the fluffing hell did you ever get your current job, when you cannot even figure out that, as DS wrote, you cannot read a tape at two places at the same time?

Please change the title in your profile to "Mainframe novice", your current title is about 42 levels over your qualifications.


Being a software engineer doesn't mean one know every thing in software industry ... This is an open forum... and I am here to learn... I know tapes are sequential data sets... But some how not able to figure out the reason in this case...
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gylbharat

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PostPosted: Fri Aug 19, 2011 3:03 pm    Post subject:
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Thanks Robert... This is useful...
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gcicchet

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Joined: 28 Jul 2006
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Location: Australia

PostPosted: Sat Aug 20, 2011 6:17 am    Post subject:
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Hi,


it's possible the dataset may have resided on DASD and now its on tape,

maybe the reason for the DISP=SHR.

VTS datasets are treated the same as tape datasets.


Gerry
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