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Extents in primary and secondary space allocation

 
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JnanaR

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Joined: 20 Jul 2009
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PostPosted: Sun Dec 27, 2009 9:15 pm    Post subject: Extents in primary and secondary space allocation
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Hello

I am a bit confused about the extents in primary and secondary space allocation.I have read the manual and it says 16 extents are possible for a PS dataset in a single volume.I want to know the number of cylinders that will be used if all the 16 extents are filled up.

for SPACE=(CYL,(200,50))


I have also checked some threads but I am not clear.


They have broken the primary allocation into 5 extents(like 200=100+40+30+20+10) but for secondary allocation they have multiplied 50 by 11.But if you go by the same rule that applies to primary
I think 50 cylinders should be divided in 11 extents..



Can anyone pls clarify why the secondary allocation is multiplied by 11 whereas the primary is not.
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Robert Sample

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PostPosted: Sun Dec 27, 2009 10:05 pm    Post subject:
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The primary allocation can be filled in as many as five extents. If there is not enough free space to complete the primary allocation within 5 extents, the allocation fails with a space error. It may be filled in 1 extent, or any number up to 5 extents. The number of potential secondary extents is 16 minus the number of extents used to fill the primary request. So if 1 extent was used for the primary, 15 remain for the secondary, but if 5 extents were required for the primary there are only 11 possible secondary extents. The maximum size of the data set is 950 cylinders (200 for the primary, 15 times 50 = 750 for the secondary extents). If the primary required 5 extents, the maximum size of the data set is 750 cylinders.

Please note that the limit can be affected by having multiple packs for the data set; a single data set can use as many as 59 volumes.
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Terry Heinze

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PostPosted: Mon Dec 28, 2009 12:09 am    Post subject:
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Isn't it also true that the introduction of SMS lifted the 16-extent maximum?
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Robert Sample

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PostPosted: Mon Dec 28, 2009 2:00 am    Post subject:
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From the z/OS V1R10.0 JCL Reference manual (link at the top of the page), section 12.59.2.1:
Quote:
When you specify a secondary quantity and the data set requires additional space, the system allocates the specified quantity:

* In contiguous tracks or cylinders, if available.

* If not available:

o If the data set does not have the space constraint relief option, in up to five extents.

o With the space constraint relief option, the system might have to allocate more than five new extents. A data set has this option only if it is SMS-managed and the data class specifies the option.

The system can allocate up to 123 extents for a data set on a volume if it is a PDSE, an HFS data set, an extended format data set, or a VSAM data set in an ICF catalog. For other types of data sets the system can allocate up to 16 extents for each data set on each volume. An extent is space that may or may not be contiguous to other space allocated to the data set. The extents for a data set include the primary quantity space and user-label space.
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Ankit Kumar Gupta

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PostPosted: Thu Jan 20, 2011 8:33 am    Post subject: Reply to: Extents in primary and secondary space allocation
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Hi Robert,

I have one question like as you mentioned
Quote:
" if 1 extent was used for the primary, 15 remain for the secondary, but if 5 extents were required for the primary there are only 11 possible secondary extents"


How will it decide how many to use from primary and then go for secondary?

like in one case, it uses 1 primary extent and then starts looking for secondary whereas in other case, it uses 5 primary and then started looking for secondary extents.

Is it based up on some contiguous space available for primary extents ?
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gcicchet

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Location: Australia

PostPosted: Thu Jan 20, 2011 9:18 am    Post subject:
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Hi,

I'm only repeating what has already been stated.

Primary quantity has to be satisfied in full before it uses secondary quantity.

ie. if primary quantity is 900 cyls, the system will attempt to allocate the space in contiguous cyls if available, if not, 900 cyls can take up to five extents.

Quote:
The primary allocation can be filled in as many as five extents. If there is not enough free space to complete the primary allocation within 5 extents, the allocation fails with a space error. It may be filled in 1 extent, or any number up to 5 extents. The number of potential secondary extents is 16 minus the number of extents used to fill the primary request.



Gerry
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Ankit Kumar Gupta

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PostPosted: Sat Jan 22, 2011 1:39 am    Post subject:
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Thank you guys....... I got it. Thanks again
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Pete Wilson

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PostPosted: Thu Jan 27, 2011 9:19 pm    Post subject:
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Check if your dataset has a DATACLAS with Space Contstraint Removal as well. That means Primary extents can be broken down into many, even more than 5. It will also do that for the first extent taken on subsequent volumes it extends to.

Just to confuse the issue further, the system will 'join' extents together if there is space available on the volume immediately after the previous extent, so you don't actually have another extent, just a larger version of the original.

If the dataset is Extended Format (also determined by it's DATACLAS)) it can have up to 123 extents per volume even if it is QSAM (PS) and can be GT 65k tracks per volume.
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