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Performance issues with GHU call

 
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cybertaurean

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PostPosted: Tue Feb 16, 2010 4:07 pm    Post subject: Performance issues with GHU call
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Hi All,

I would like to know if there are any known performance issues with GHU call used for DB repositioning. Take the below example -

Code:
call 'CBLTDLI' for xrst
if xrst
   call 'CBLTDLI' for GHU
end-if

read-db till end (GN) save off the key-seg

             certain conditions met, reposition using key-seg (GHU)
             delete the segment
end-read

The above operation is performed for a huge DB. Is there any performance hit that could happen in this context where we read using GN till EOF and in between use GHU to reposition back and DLET.

I have done few optimizations in Cobol-DB2 areas where we substituted the DB2 table with an unload of it treated as a GSAM and had a program read the file and make changes to DB as required. Can I use the same logic in IMS -

Code:
take an unload of the entire IMS-DB, treat it as GSAM
process the records one by one, and writing only the ones I want to retain to an output file.
Do a LOAD REPLACE (not sure how this can be done in IMS-DB)


Please let me know if this can be implemented.


Regards,
Sumesh
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Sandy Zimmer

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PostPosted: Tue Feb 16, 2010 5:37 pm    Post subject:
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Once again - IMS is NOT DB2!

First of all, GSAM is only 1 level. How many levels are in your database?

Are you running this process as DLI or BMP? What determines what gets deleted?
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cybertaurean

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PostPosted: Tue Feb 16, 2010 6:09 pm    Post subject: Reply to: Performance issues with GHU call
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Hi Sandy,

Thanks for the response.

The DB in context contains a single segment and the logic for deleting the records is something that is based on the date fields in fields present within the segment. There is no dependency between the data present in different segments (bottomline, segments can be processed individually).

Let me know if you think the unload-process-load would be a good option instead of direct processing.

Regards,
Sumesh
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Sandy Zimmer

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PostPosted: Tue Feb 16, 2010 6:38 pm    Post subject:
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Can you post your DBD?
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cybertaurean

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PostPosted: Wed Feb 17, 2010 10:22 am    Post subject: Reply to: Performance issues with GHU call
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Code:
DBD   NAME=A,ACCESS=(PHIDAM,OSAM),               
      VERSION='06/18/0309.39'                         
SEGM  NAME=SEGLA,BYTES=1000,PTR=(TB),               
      PARENT=0,RULES=(LLL),COMPRTN=(DPIEXIT,DATA,INIT),
      DSGROUP=A                                       
FIELD NAME=(AKEY,SEQ,U),BYTES=57,START=1,TYPE=C   
FIELD NAME=ITEM1,BYTES=2,START=1,TYPE=C             
FIELD NAME=ITEM2,BYTES=20,START=3,TYPE=C           
FIELD NAME=ITEM3,BYTES=8,START=23,TYPE=C           
FIELD NAME=ITEM4,BYTES=4,START=46,TYPE=C           
FIELD NAME=ITEM5,BYTES=1,START=50,TYPE=C           
FIELD NAME=ITEM6,BYTES=6,START=51,TYPE=P                   
FIELD NAME=ITEM7,BYTES=1,START=57,TYPE=C                   
FIELD NAME=ITEM8,BYTES=20,START=75,TYPE=C                   
FIELD NAME=/SX01,BYTES=8,START=1                               
LCHILD NAME=(SEGLB,LDBBX),PTR=INDX                         
XDFLD NAME=LASEC,SEGMENT=SEGLA,                               X
      SRCH=(SUBRCRTN),                                        X
      SUBSEQ=(/SX01)                                           
DBDGEN                                                         
FINISH                                                         
END                                                           
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Sandy Zimmer

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PostPosted: Wed Feb 17, 2010 7:32 pm    Post subject:
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You have a logical child segment defined. What is your "delete" criteria and how will this logical child segment be affected?
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Sandy Zimmer

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PostPosted: Wed Feb 17, 2010 8:25 pm    Post subject:
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I would make this as efficient as possible. The best way to do this would be to do a read only on the database. Make sure that you have an image copy or use the current database as the image copy.

You will have 2 sequential files defined - one for segments you are keeping and one for segments you are deleting.

You will write to one of the 2 files based on whether they stay or get deleted - we will call them KEEP File and DELETE File.

You will NOT do any updates to the database you are reading - make your PSB read only.

Then do a delete/define of a TEST database. You will use your KEEP File to LOAD this database. Once verified as correct, you will image copy this TEST database to PRODUCTION.

Do not throw away the DELETE file. Both of your sequential files can be used as audit trails.
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cybertaurean

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PostPosted: Thu Feb 18, 2010 4:17 am    Post subject: Reply to: Performance issues with GHU call
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Hi Sandy,

The LCHILD is the secondary index for the DB. Wouldn't that be handled automatically when we delete from DB?

Say, I create two files RETAIN and DELETE from the file, how can I make use of the RETAIN file to load the DB? Is there a utility or do I have a write a PROGRAM with the RETAIN file as a GSAM since the creation of RETAIN has been done programatically?.

Could you also please explain the delete/define + IMAGECOPY part that you mentioned in the end?. This DB is already an existing one.

Appreciate your great help with this.

Regards,
Sumesh
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Sandy Zimmer

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PostPosted: Thu Feb 18, 2010 4:26 am    Post subject:
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OK - first of all, any delete of a root or parent segment - as I have said many times before - deletes any child segments all the way down the chain.

You need to thorougly test any procedure before implementation. I have done many projects with the same scenario that I described to you. You must have a DBA that can help along the way - right? Anyway, when designing this system, you need to consider how the delete of your root will affect any other system. I would not use the GSAM. Since you are dealing with a large volume of data, you want your system to run as fast as possible.

The database that you use as the final result - after multiple testing - should be your image copy.

Your delete/define/init of your KEEP database is an easy task for your tech services group. Any secondary index from this new database is also an easy task for your tech services group..
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cybertaurean

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PostPosted: Thu Feb 18, 2010 4:52 am    Post subject: Reply to: Performance issues with GHU call
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Hi Sandy,

In case I were to treat the file as non-GSAM, wouldn't that complicate things when it comes to implementing checkpoint restart? Say, the program abends, I would probably keep track of the number of records read and then read through till the count that was registered when the last successful checkpoint was taken and then resume processing. Please see the stub below -

Code:
?xrst
     read_DELETE_FILE process_count times
else
     read_DELETE_FILE
?end
perform main_loop until EOF

:main_loop
     delete the record from IMS
     ?OK
            process_count ++
            checkpoint
     ?end
:end

Would the above logic be better than having IMS take care of checkpointing?

Regards,
Sumesh
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Sandy Zimmer

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PostPosted: Thu Feb 18, 2010 5:00 am    Post subject:
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Nope - if you are running BMP - take the current image copy of the production database.

Run against the thing offline - as a DLI. There is absolutely NO REASON to run checkpoint restart if you are doing this process as a "read only". You are not updating a thing. If the program abends - and why would it - you have not updated anything. It will run fast - believe me. You are just writing to 2 sequential non-IMS files.
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cybertaurean

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PostPosted: Thu Feb 18, 2010 5:05 am    Post subject: Reply to: Performance issues with GHU call
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Great!!!!!

One final question (and I guarantee this will the last icon_biggrin.gif).

Instead of having the RETAIN file LOAD the DB, wouldn't it be better if I used the DELETE file to DLET data from DB. The reason being that, the DELETE count will be lesser compared to the RETAIN count.

Thoughts???
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Sandy Zimmer

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PostPosted: Thu Feb 18, 2010 5:16 am    Post subject:
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NOPE!!! Absolutely not! What you are doing in a simple process is cleaning up the database. In addition, you are doing what we term as as "reorg" . You will be freeing up now unused space - root anchor points - forward - backward pointers and all that good stuff. Take my word - you will be accomplishing many tasks with your process. Make sure that your DBA group understands what you will be doing - they should really appreciate your innovation. Just keep us posted on how you are doing. Just do this FIRST against your test databases.

So the process should be - GN database record - either write to KEEP or DELETE file. Check out those sequential files - sequential files are fast and easy no matter how large - once you finish your audit - check with your user - then do the rest.
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cybertaurean

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PostPosted: Thu Feb 18, 2010 5:26 am    Post subject: Reply to: Performance issues with GHU call
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Thanks a lot!!!!

Will keep you posted....
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Sandy Zimmer

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PostPosted: Thu Feb 18, 2010 5:34 am    Post subject:
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You are very welcome! I like to keep any process simple and easy to maintain - this process won't cost anyone anything - you may need to run it "after hours", but it will work and you will accomplish many tasks with one process. Don't worry about asking for further help!
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