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  1. #1
    Join Date
    Sep 2003
    Posts
    36

    Unanswered: Negative Compression Ratio

    Dear Freinds,
    Can anyone help me with what is Negative Compression ratio in mainframe DB2 V 7.
    Thanks
    Sahana

  2. #2
    Join Date
    Feb 2002
    Location
    Germany
    Posts
    141
    Sahana,

    if you are talking about column PAGESAVE on SYSIBM.SYSTABLEPART, you can have a negative value when your tablespace after compression has more pages than before compression. This occurs seldom, by small tablespaces or tablespaces where compression isn't possible (ex. binary data).
    An example: you have a tablespace and all the data fits in one page. Without compression you get 3 pages (header, space map page and data page). With compression, you would get 4 pages (header, space map page, data dictionary, data pages).

    HTH.
    Rodney Krick

  3. #3
    Join Date
    Sep 2003
    Posts
    36
    Originally posted by RKrick
    Sahana,

    if you are talking about column PAGESAVE on SYSIBM.SYSTABLEPART, you can have a negative value when your tablespace after compression has more pages than before compression. This occurs seldom, by small tablespaces or tablespaces where compression isn't possible (ex. binary data).
    An example: you have a tablespace and all the data fits in one page. Without compression you get 3 pages (header, space map page and data page). With compression, you would get 4 pages (header, space map page, data dictionary, data pages).

    HTH.
    Thanks RKrick for your fast response......, I have a quick clarification.........In your example quoted.....if the page size increase from 3 to 4 after compression, then the PAGESAVE of the tablespace will have -1 value as PAGESAVE in SYSIBM.SYSTABLEPART_HIST
    ...right....Also in what way this value helps in performance tuning area .....Thanks in Advance......
    Kindly clarify
    Sahana

  4. #4
    Join Date
    Feb 2002
    Location
    Germany
    Posts
    141
    Sahana,

    why do you use compression? To get more data on your bufferpools. If you're becoming negative compression ratios, you're not achieving it! Remember also, that the compression dictionary will be loaded into your DBM1 Adress Space, so you are wasting memory.

    HTH.
    Rodney Krick

  5. #5
    Join Date
    Sep 2003
    Posts
    36
    Originally posted by RKrick
    Sahana,

    why do you use compression? To get more data on your bufferpools. If you're becoming negative compression ratios, you're not achieving it! Remember also, that the compression dictionary will be loaded into your DBM1 Adress Space, so you are wasting memory.

    HTH.
    Again thanks for your quickest relpy.....
    I got what u r trying to convey......one clarification is what will be the value of the pagesave when there is an increase in the datapage value from 3 to 4 after compression ......ie what is the page value now after the above increase....

    Regards,
    sahana

  6. #6
    Join Date
    May 2003
    Location
    USA
    Posts
    5,737
    According to the manual PAGESAVE is a percentage, not the number of pages. So a negative 1 percent is not significant, but obviously does not justify compression on that table.

    Your decision whether to compress that table may depend on if the existing data is representative of what data will be stored in that table in the future. I assume this value is updated via runstats or some other utility, so it is not always current.

  7. #7
    Join Date
    Sep 2003
    Posts
    36
    Thanks Marcus , RKrick for your reply to my query on "Negative Compression Ratio"
    That was really helpful


    Sahana

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