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Confused over Extents allocation

 
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barthwalsamarth

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Joined: 03 Aug 2006
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PostPosted: Wed May 14, 2008 11:42 am    Post subject: Confused over Extents allocation
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I was going through a text which says "Extent is contiguous memory location. Only 16 extents are possible for a physical sequential dataset in a volume"
That means
Total Space = 1*primary + 15 *secondary so a total of 16.

Now next thing mentioned is "Primary and secondary space request has to be met within 5 extents" What does this mean then?
Also if I code
Code:
(SPACE=TRKS,(100,100))
then what will be the size of each extent?

Please help with this allocation concept.

Thnaks
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expat

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PostPosted: Wed May 14, 2008 11:45 am    Post subject:
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Quote:
(SPACE=TRKS,(100,100))


It means that the 100 tracks specified will be allocated in one extent if sufficient contiguous space exists, if not, the 100 tracks can be allocated in up to 5 seperate extents.
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gcicchet

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PostPosted: Wed May 14, 2008 11:48 am    Post subject:
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Hi,
see linkhttp://ibmmainframes.com/viewtopic.php?t=26006&highlight=space+gerry


Gerry
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rajatbagga

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Joined: 11 Mar 2007
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Location: india

PostPosted: Wed May 14, 2008 11:57 am    Post subject: Re: Confused over Extents allocation
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barthwalsamarth wrote:

Now next thing mentioned is "Primary and secondary space request has to be met within 5 extents" What does this mean then?


It means that 500 Tracks were allocated for the code:
Code:
SPACE=TRKS,(100,100)


barthwalsamarth wrote:

Also if I code
Code:
(SPACE=TRKS,(100,100))
then what will be the size of each extent?


Well the size of track depends on the Device Type
For eg.

1.) For the DEVICE TYPE = 3390 number of bytes in one TRACK is 56664 BYTES.

2.) For the DEVICE TYPE = 3390 number of TRACKS in one CYLINDER is 15 TRACKS.

3.) BLOCKS are INDEPENDENT of the device type but it depends on the input files DCB parameter.

I hope this helps,

Regards,
Rajat
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expat

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PostPosted: Wed May 14, 2008 12:09 pm    Post subject:
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Quote:
It means that 500 Tracks were allocated for the code:
Code:
SPACE=TRKS,(100,100)

NO it does not. Please read the previous responses before giving wrong information.
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rajatbagga

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PostPosted: Wed May 14, 2008 12:09 pm    Post subject:
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Hi expact,

expat wrote:
Quote:
(SPACE=TRKS,(100,100))


It means that the 100 tracks specified will be allocated in one extent if sufficient contiguous space exists, if not, the 100 tracks can be allocated in up to 5 seperate extents.


Well expact i think if one extent is not sufficient then the 100 tracks can be allocated in up to 4 seperate extents.

Please let me know if I am wrong...

Regards,
Rajat
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rajatbagga

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PostPosted: Wed May 14, 2008 12:10 pm    Post subject:
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Ok ,

I got the answer
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gcicchet

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PostPosted: Wed May 14, 2008 12:11 pm    Post subject:
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Hi,
by the way SPACE=TRKS will give you a JCL error it should be TRK


Gerry
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expat

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PostPosted: Wed May 14, 2008 12:14 pm    Post subject:
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What I am saying is ...........

If there is an area of free space that is greater than or equal to 100 tracks, then the 100 track space request will be satisfied in one physical extent of 100 tracks in size.

If not, the operating system will then see if the 100 tracks can be allocated in up to five extents, and if so allocate the space request in up to five extents.

So 100 tracks can be made up of 50 tracks + 30 tracks + 20 tracks - 3 extents
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mmwife

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Joined: 30 May 2003
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PostPosted: Wed May 14, 2008 7:55 pm    Post subject:
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The same goes for the 2ndary space, but the max extents allowed for a single vol DS is 16. So using expats example of a 5 extent primary, you are limited to 11 2ndary extents max. If you need more than 1 extent for any 2ndary the max is reduced further.

BTW, if you alloc across multi vols you get 16 additional 2ndary extents on each vol after the 1st. Same rules apply.
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