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How to Calculate Control Interval Size

 
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n459760

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Joined: 13 Oct 2009
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PostPosted: Fri Mar 05, 2010 12:31 am    Post subject: How to Calculate Control Interval Size
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Hi,

I did research and read through VSAM Demystified again before posting this but I am getting incorrect output.

How to calculate or what should be the optimum control interval sizes of data and index files for a VSAM file with LRECL of 94 and assuming a record count of 500,000,000 (.5 billion) with a monthly percentage increase of 3% of records.

I checked with some of the formulae's but I am getting conflicting results. Your thoughts and inputs are really appreciated.
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Robert Sample

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PostPosted: Fri Mar 05, 2010 12:38 am    Post subject:
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Will the file be accessed randomly or sequentially or a mixture of access?

How often will the file be purged (3% monthly is 36% annually if not compounded or 180 million extra records per year -- you cannot keep that growth rate up forever)?

What is the key length? Index CI size depends at least partly on key length.

I'm assuming you want a KSDS here, although you don't specify which of the several types of VSAM files you're dealing with.

In general, for sequential access 18432 data CI size works well for almost any conceivable record size. However, if you're planning on accessing the file online randomly, the large CI size may cause issues with locking and buffer space. The answers you're looking for are going to involve trade offs between the various factors.
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n459760

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PostPosted: Fri Mar 05, 2010 12:57 am    Post subject:
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Hi Robert,

Thank you for your quick reply:

How often will the file be purged (3% monthly is 36% annually if not compounded or 180 million extra records per year -- you cannot keep that growth rate up forever)?

- The file will be deleted and defined daily with the data getting loaded from a BDAM file.

What is the key length? Index CI size depends at least partly on key length.

- The key length is 12.

Yes, its KSDS. Sorry I did not specify this in original post. The file will be accessed randomly and then sequentially to get all records of that key.

I am not able to recall or find but I think I did read somewhere that while defining the file we must take into account the increase in size of the file so as to account of CI/CA splits. Additionally control interval size definition should also take into account these increases.

I was trying to get a consensus here regarding any thumb rule/approximations/generalization that may exist when we define a KSDS file specially with respect to the CISZ.

I did use the 18432 data CI but I am getting error VSAM error 28. I must also include here that this is an extended format VSAM.
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Robert Sample

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PostPosted: Fri Mar 05, 2010 2:03 am    Post subject:
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Quote:
The file will be accessed randomly and then sequentially to get all records of that key.
Unless you are talking about alternate indexes, you do need to be aware that a primary key must be unique in a VSAM KSDS -- there will be no other "records of that key". If you were talking about a compound key, where you're doing generic retrievals, I could see it but you haven't mentioned a lot about the file.

CI/CA splits will be irrelevant if you are defining the file daily -- unless you've got some very unusual update patterns. On the other hand, deleting defining and loading this file every day is going to take quite some time.

Are you working with your site support group to define this file? 500 million 94 byte records is going to run 40 gigabytes or 14 to 15 3390-3 disk packs, which would require advance planning with the storage management people usually. Since the limit for a VSAM file is 4 GB unless it is extended format, you didn't really tell us anything by mentioning that it is extended format -- we assumed that.

How many volumes are you defining for the data component? Are they in a storage pool of their own, or shared with other files? Are you having problems getting enough space -- a 28 return code can mean you've run out of disk packs (or extents) and the file still needs to grow some.

My handy little Excel VSAM tuner tells me 26624 is probably your best data CI size (as long as there's an online pool big enough for that CI size). Index CI size won't matter since the rule of thumb says even a 512 byte CI size will hold all the data pointers.

When I said "purged" in my earlier post, I meant records removed -- permanently -- from this file, not just a delete / define of it. 180 million new records a year is adding 16 gigabytes of data or 6 new 3390-3 disk packs each year. Sooner or later you're going to hit the 59 disk pack limit for files, unless records are removed from the file sooner or later. But that is an application design issue, not a technical issue.
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raghavmcs

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PostPosted: Fri Mar 05, 2010 3:31 am    Post subject:
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Hello Robert,

I usually visit the expert answers on this forum,I really liked your this post backed up with statistics and full of experience.
I am wondering if you could share your handy excell on this forum.thanks
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Robert Sample

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PostPosted: Fri Mar 05, 2010 3:43 am    Post subject:
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I'll look into it. I've actually got 2 I've written over the years -- the one I used today allows input of record size and gives unused bytes and percentage for each possible CI size to help with tuning. The other one, which I don't seem to have on this machine, estimates file sizes based on record size, key length, free space figures, and number of records -- for each possible CI size.
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n459760

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PostPosted: Fri Mar 05, 2010 7:16 pm    Post subject: Reply to: How to Calculate Control Interval Size
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I would also be greatly indebted if you can share how you arrived at 26624 for data CI. Thank you.
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enrico-sorichetti

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PostPosted: Fri Mar 05, 2010 7:26 pm    Post subject: Reply to: How to Calculate Control Interval Size
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1024 * 26
but I wonder why not 27648 ( 1024 * 27 ) the best half track approximation
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