Extents in primary and secondary space allocation

JnanaR · New User Joined: 20 Jul 2009 Posts: 23 Location: Mumbai

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.

Robert Sample · Posted: Sun Dec 27, 2009 10:05 pm

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.

Terry Heinze · Posted: Mon Dec 28, 2009 12:09 am

Isn't it also true that the introduction of SMS lifted the 16-extent maximum?

Robert Sample · Posted: Mon Dec 28, 2009 2:00 am

From the z/OS V1R10.0 JCL Reference manual (link at the top of the page), section 12.59.2.1:

Ankit Kumar Gupta · New User Joined: 08 Jan 2011 Posts: 16 Location: India

Hi Robert,

I have one question like as you mentioned

gcicchet · Posted: Thu Jan 20, 2011 9:18 am

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.

Ankit Kumar Gupta · New User Joined: 08 Jan 2011 Posts: 16 Location: India

Thank you guys....... I got it. Thanks again

Pete Wilson · Posted: Thu Jan 27, 2011 9:19 pm

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.