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Paulo Moreira · Beginner Joined: 15 Sep 2004 Posts: 2 Topics: 1

Anyone know how do I determine if a Vsam file need to be Reorg. ? I mean, the amount of fragmented space, instead of the space available.

Thanks
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Paulo Moreira

kolusu · Posted: Thu Sep 16, 2004 5:21 am Post subject:

Paulo Moreira,

There is no rule of thumb as to when to reorganize a vsam cluster. It is a good practise to re-org the vsam cluster when there are too many CI/CA splits. If a VSAM file is constantly updated with new records, they will keep causing splits. This has performance implications.

As CA splits occur, the file is going to expand in size until it could occupy significantly more space than required. CA splits will create new free space that can be useful if the file receives clustered inserts. Once the CA split occurs, the extra overhead processing the file may be small. A bad design of key structure also has lot of performance implications.

Hope this helps...

Cheers

Kolusu
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Kolusu
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Paulo Moreira · Beginner Joined: 15 Sep 2004 Posts: 2 Topics: 1

Kolusu, thank you.

But, how do I know the size and percent fragmented space allocated for a vsam file ?
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Paulo Moreira

kolusu · Posted: Thu Sep 16, 2004 12:05 pm Post subject:

Paulo Moreira,

Just do a Listcat on the vsam cluster and look at the listing. Post the listing here and may be we can calculate it

Kolusu
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MikeBaker · Beginner Joined: 04 May 2004 Posts: 96 Topics: 9

Paulo Moreira,

However, do not assume that a re-org'ed file will necessarily perform better than a file with CI/CA splits. It may actually perform worse.

The reason why a split occurs in the first place is because space is required at that particular place in the file. And once a split has occurred, if extra records are also needed at that same place in the file, the space is now there for them; you won't need to split again. If you were to do a reorg at this point, you would then take away space from the very place where it is needed. Not clever.

One rule of thumb is that if it is a small VSAM file (with few inserts) that is not used heavily, then don't be too concerned about tuning it. If however, you have lots of VSAM files in your shop, and a significant amount of processing occurs with them, then tuning is very important, and the returns can be dramatic. And tuning is not a straight forward exercise; it takes a bit of time to get it right. And it doesn't just simply mean re-org'ing all the time either. Alot of it has to do with getting the cluster definition right in the first place. Easier said than done.

The world's foremost expert on VSAM is a fellow known as Ron Ferguson, who runs Mainstar. Try and go on one of his courses if you ever get the chance.