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Multiple Disk Usage

 
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saurabhkhandelwal

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Joined: 19 Mar 2008
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PostPosted: Fri Dec 19, 2008 11:50 am    Post subject: Multiple Disk Usage
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Hi ,
I have a doubt ..
while writing JCL sometimes we allocate space to a dataset created as :
//JSTEP2 EXEC PGM=SORT
//SORTIN DD DSN=L.L8033.DGFILE.SIBSORT,DISP=OLD
//SORTOUT DD DSN=L.L8033.DGFILE.MERGSIBL,
// DISP=(NEW,CATLG,DELETE),
// DCB=(RECFM=FB,LRECL=430,BLKSIZE=0),
// UNIT=(DISK,3),SPACE=(CYL,(2000,500),RLSE)
//SYSOUT DD SYSOUT=*
//SYSIN DD *
OPTION COPY
/*

So what is the significance of using multiple disk and how the space is being allocated to the dataset created in this case .

Regards ,
Saurabh Khandelwal
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dick scherrer

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PostPosted: Fri Dec 19, 2008 11:58 am    Post subject:
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Hello,

The posted UNIT specification allows the allocation to span over 3 physical volumes.

There is extensive information about the space parameter here:
http://publibz.boulder.ibm.com/cgi-bin/bookmgr_OS390/handheld/Connected/BOOKS/iea2b650/12.55?

That link goes directly to the space parameter. In the same book, is all of the other jcl info including the UNIT specification.

If you find something in the documentation that is not clear, post what you found and your question about it. Someone will be able to clarify.
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saurabhkhandelwal

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PostPosted: Fri Dec 19, 2008 12:32 pm    Post subject: Multiple disk usage
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Hi.
My concens was when we are allocating three disk to a dataset ,
so whatever the number if cylinders are specified for it , is it distributed among the three volumes or it is defined specifically for all the three volumes..ie ( cyl * 3 ) ..

dick scherrer wrote:
Hello,

The posted UNIT specification allows the allocation to span over 3 physical volumes.

There is extensive information about the space parameter here:
http://publibz.boulder.ibm.com/cgi-bin/bookmgr_OS390/handheld/Connected/BOOKS/iea2b650/12.55?

That link goes directly to the space parameter. In the same book, is all of the other jcl info including the UNIT specification.

If you find something in the documentation that is not clear, post what you found and your question about it. Someone will be able to clarify.
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dick scherrer

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PostPosted: Fri Dec 19, 2008 12:36 pm    Post subject:
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Hello,

You might allocate the total mentioned in the space parameter and it might be allocated across 3 dasd devices. You would not get the allocation times 3.
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mmwife

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PostPosted: Fri Dec 19, 2008 7:37 pm    Post subject:
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Another point to remember when you're calculating space is that you can get up to a maximum of 500 CYL of 2ndary space (from your example) 16 times for EACH of the additional vols, as well as the usual max of 15 times 500 for the 1st vol.

Doing the math for the JCL you provided, SORTOUT dataset could contain a max of 25,500 CYL:
Code:


 2000      PRIMARY
 7500      VOL 1   2NDARY
16000      VOL 2-3 2NDARY
--------   -------------------
25500      TOTAL
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saurabhkhandelwal

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PostPosted: Mon Dec 22, 2008 10:32 am    Post subject:
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Hi ..
So according to you , as per my example [ (2000,500) ] ,
The Primary cylinders will be allocated to three volumes and in case of secondary cylinders the distribution will be :
15*500 = 7500 , for 1st CYL.
16*500 = 8000 , for 2nd CYL
16*500 = 8000 , for 3rd CYL .

This is what you are saying and could you please clarify me the reason of such calculations while assigning maltiple disks.
And what is the maximum number of multipe disk that can be assigned to a dataset.

mmwife wrote:
Another point to remember when you're calculating space is that you can get up to a maximum of 500 CYL of 2ndary space (from your example) 16 times for EACH of the additional vols, as well as the usual max of 15 times 500 for the 1st vol.

Doing the math for the JCL you provided, SORTOUT dataset could contain a max of 25,500 CYL:
Code:


 2000      PRIMARY
 7500      VOL 1   2NDARY
16000      VOL 2-3 2NDARY
--------   -------------------
25500      TOTAL
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dick scherrer

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PostPosted: Mon Dec 22, 2008 10:57 am    Post subject:
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Hello,

Quote:
The Primary cylinders will be allocated to three volumes
It will not use 3 devices for the primary allocation. It will depend on what space is available when the allocation is attempted as to whether the primary can be met in one extent. (Corrected 12/22) - d.)

Quote:
in case of secondary cylinders the distribution will be :
15*500 = 7500 , for 1st CYL.
16*500 = 8000 , for 2nd CYL
16*500 = 8000 , for 3rd CYL .
I am confused by the reference to 1st, 2nd, and 3rd CYL. The 15/16/16 is also not clear. Keep in mind that specifying 3 volumes may be used does not request the space * 3. It only allows the original space request to be spread across multiple volumes.


Quote:
And what is the maximum number of multipe disk that can be assigned to a dataset.
Suggest you talk with your storage management people or system support people for the local rules that have been implemented.
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gcicchet

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PostPosted: Mon Dec 22, 2008 11:28 am    Post subject:
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Hi Dick,

when allocating space across multiple volumes, the primary space is allocated to 1st volume only, 2nd and 3rd volume will use the secondary space allocation if required.

so vol1 will use 1 primary and 15 secondary extents
vol2 will use 16 secondary extents
vol3 will use 16 secondary extents

That's assuming each primary and secondary space allocation uses 1 extent.

Gerry
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gcicchet

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PostPosted: Mon Dec 22, 2008 11:33 am    Post subject:
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Hi Dick,

I think the reference to 1st 2nd and 3rd cyl should be VOL1 VOL2 & VOL3.


Gerry
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UmeySan

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PostPosted: Mon Dec 22, 2008 3:37 pm    Post subject:
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Hi gcicchet !

If i remember, when using multiple volumes for allocating space, first the needed space for the DSN is allocated at the first volume. Primary first, then secondary. Then, when more space is needed, alloction is done to the next volume. And here, the primary space is also allocated first.

When using multiple volumes, make shure, that the size of the primary space is available on every disk you use.

If I'm mistaken and there is somthing new about that, perhaps SMS does something automatically, i think enrico-sorichetti will correct me.
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expat

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PostPosted: Mon Dec 22, 2008 4:11 pm    Post subject:
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DASD space allocations .........

Gerry is correct in saying that only the secondary allocation value will be used on volumes other than the first.

So assuming that all allocations can be satisfied in a single extent, 1st volume will contain 1x200 + 15*500 = 9,500 cyinders. 2nd and other volumes = 16*500 = 8000 cylinders.

The maximum number of cylinders that you may allocate using (2000,500) over three volumes is 25,500.

However, each and every allocation, primary or secondary may be satisfied in up to 5 physical extents, which will seriously reduce the amount of space that will be allocated.

e.g. primary allocation of 2000 cylinders uses 5 physical extents, 1st secondary allocation of 500 cyinders uses 5 physical extents, 2nd secondary allocation of 500 cylinders uses 5 physical extents. This means that you have used 15 physical extents and only allocated 3,000 cylinders. And given the same scenario on volumes two and three where each allocation uses 5 physical extents we have 3,000 + 1,500 + 1,500 = 6,000 cylinders allocated.

The more fragmented your DASD, the less chance of getting large contiguous physical extents.

Unfortunately a lot of wannabe storage people think that with the advent of virtual DASD the need to reorganise volumes has disappeared. This is not the case as although the DASD is virtual it is still mapped as if it were physical DASD including all of the old constraints of classic DASD.
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UmeySan

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PostPosted: Mon Dec 22, 2008 4:52 pm    Post subject:
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Hi !

I apologize !!!

Only secondary allocations will be used on volumes other than the first.
I just investegated the subject at www.ibm.com.

Sorry, i have to update some essentials.

Have a nice x-mas !!!
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dick scherrer

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PostPosted: Mon Dec 22, 2008 8:33 pm    Post subject: Reply to: Multiple Disk Usage
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Quote:
when allocating space across multiple volumes, the primary space is allocated to 1st volume only, 2nd and 3rd volume will use the secondary space allocation if required.
Yup, got ahead of myself icon_redface.gif Thanks for the correction.

I've edited my mis-information above as well.

d
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