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Whats mounting and demounting in tapes

 
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sudhanaveenkumar

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PostPosted: Tue Jul 27, 2010 12:38 pm    Post subject: Whats mounting and demounting in tapes
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Hi..


can any one of you tell me whats this mounting and demounting for tapes.

Why is this required.

In case of tapes.. while defining new tape we use unit=(tape,defer)... why is this defer user?///


Thanks
naveen
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sudhanaveenkumar

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PostPosted: Tue Jul 27, 2010 12:40 pm    Post subject: Whtas the adv of storing DCB of vsam in catalog
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Hi all...

in case of non vsam files, DCB is stored in VTOC.For VSAM-- ITS STORED IN CATALOG.Whats the advantage of storing DCB in catalog?


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Naveen
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expat

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PostPosted: Tue Jul 27, 2010 12:55 pm    Post subject:
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One can only assume that these are interview or homework questions.
Moved to this forum.
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Robert Sample

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PostPosted: Tue Jul 27, 2010 4:07 pm    Post subject:
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3480 / 3490 / 3590 tapes are physical media about the size of an 8-track tape cartridge (if you ever saw one of those). Tapes are cheap while tape drives to access the data on tapes are much more expensive. Tape libraries therefore allow access to many, many more tapes than tape drives (most libraries will hold tens of thousands of tapes while having some 4 to 32 tape drives). Mounting a tape is the process of finding it and loading it into a tape drive so the data can be read (or written). Demounting (or unmounting to use the more common term) is the process of rewinding the tape and ejecting it from the tape drive to it can be returned to its storage spot. JES will generally mount tapes when a job starts up; defer in the JCL causes the tape mount to be done when the tape is needed, not at job start.

The first problem with your second post is that VSAM does not store DCB data in the catalog. A VSAM file does not have a DCB so there is no data to be stored anywhere. The second problem is where did you hear that this data is stored in the catalog ? All the catalog has is the data set name, volume the data set resides on, and the type of media the volume is. File details are stored in the VTOC or tape header not the catalog.
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Gousiya Mulla

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PostPosted: Tue Jul 27, 2010 4:21 pm    Post subject:
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I heard several times from the support guys that 'Tapes need to mount' (physical tapes) . In this case is this a physical movement or through running a job.

Any additional information on this will be very useful !!
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Robert Sample

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PostPosted: Tue Jul 27, 2010 4:40 pm    Post subject:
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It typically is done through a job which requests a particular tape (or no particular tape for an output, or scratch, volume). The mount is physical movement of the tape from wherever it is located to the tape drive and insertion of the tape into the tape drive so it can be read or written.
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sudhanaveenkumar

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PostPosted: Tue Jul 27, 2010 6:25 pm    Post subject:
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Hi Robert,

Many Thanks for your prompt reply.

I am clarified for the first question.

Regarding VSAM question: I have read Murah Jcl book.It says VSAM is catalog driven and all the details of file are stored in catalog(Including DCB).

As per your stmt above(i.e . File details are stored in the VTOC or tape header not the catalog), if VSAM file DCB details are stored in VTOC,then why cant we access an uncatalogged VSAM file and why every vsam file has to be catalogged?


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Naveen[/b]
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Robert Sample

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PostPosted: Tue Jul 27, 2010 6:54 pm    Post subject:
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Quote:
I have read Murah Jcl book.It says VSAM is catalog driven and all the details of file are stored in catalog(Including DCB).
Actually, you have read no such thing -- VSAM files have ACB (access control block), not DCB. You are completely WRONG to refer to DCB with VSAM files. DCB has LRECL -- VSAM has record length; DCB has BLKSIZE -- VSAM has CISIZE; the differences go on and on and on.

I think some of your confusion comes from the fact that data about VSAM files are stored in the VVDS (VSAM volume data set) which is volume specific but is considered part of the ICF catalog. Since VSAM definition data is stored in the VVDS extended format component of the catalog, it makes no sense to talk about uncataloged VSAM files.
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Gousiya Mulla

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PostPosted: Wed Jul 28, 2010 9:09 am    Post subject:
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Quote:
The mount is physical movement of the tape from wherever it is located to the tape drive and insertion of the tape into the tape drive so it can be read or written.


In this scenario , how it works for the Virtual tapes . Recently we had Virtual tapes implementation , in which all the physical tapes have been replaced by the Virtual ones.

I think in this case there will not be any mounting or tapes onsite/offsite issues ...

Correct me if I am wrong ...!!
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dick scherrer

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PostPosted: Wed Jul 28, 2010 9:29 am    Post subject:
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Hello,

Virtual Tape (or Vtape) creates logical tape volumes on dasd. Virtual Tape Drives are defined to process these virtual tapes (if you look at the jes log of 2 jobs that use "real" tapes and virtual tapes, you will notice that the tape drive numbers (UCB) for the virtual tapes and the real tapes are not in the same series.. These "tapes" are still managed by the tape management system (i.e. CA-1). They are just created in different tape pools within the tape management system.

The normal "mount" messages still appear on the console (unless they have been inhibited).

Virtual tapes are usually copied onto 3590 (FAT) tapes which may be taken offsite for storage.

And no, the virtual tapes themselves are not physically taken offsite for storage.
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Gousiya Mulla

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PostPosted: Wed Jul 28, 2010 10:18 am    Post subject:
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Thanks Dick and Robert !
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sudhanaveenkumar

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PostPosted: Wed Jul 28, 2010 11:14 am    Post subject:
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Many Thanks Robert and Dick.

I am clarified for the second question as well now.


Thanks
Naveen
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Bill Dennis

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PostPosted: Wed Jul 28, 2010 8:16 pm    Post subject:
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Some virtual tape implementations archive virtual volumes to physical tapes to free disk space. A recall of this vitrual volume will require a physical mount on the back end (robotics?) and lead to a delay of a few minutes. The backend activity is hidden from your Job.
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