buffer hit ratio% negative, rollback wrap
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  1. #1
    Join Date
    May 2000
    Location
    fremont ca
    Posts
    182

    Angry

    I install sportlight (quest software ) , oracle management server performance dignostic.
    I am trying to monitor my production database and found
    buffer hit%;
    Sportlight shows 0% to 50%
    (Oem)Performance manager- 81%
    (Oem)memory cache hits , buffer cache hit% negative.

    My database has some serious problems, Now can I get correct buffer hit%.

    If you do not specify optimal size in rollback segment what will happen ? What is the meaning of rollback wrap, undo header ? Does it effact degrad performance. My rollback shrink is 0.

    Please help


  2. #2
    Join Date
    Aug 2001
    Location
    Waterloo, On
    Posts
    547
    I can't fathom how buffer hit % can be negative.

    You calculate Buffer cache hit ratio as follows:
    Code:
    Select name, value from V$SYSSTAT where name in ('db block gets', 'consistent gets', 'physical reads');
    Now Calculate hit ratio in % by:

    (db block gets+ consistent gets - physical reads/db block gets + consistent gets)*100

    Did you run the database for several hours before taking the hit ratio?

    If you do not specify optimal size in rollback segment what will happen ? What is the meaning of rollback wrap, undo header ? Does it effact degrad performance. My rollback shrink is 0
    If you do not specify optimal size while creating roll back segments, they will not shrink. There is a chance of them getting elongated out of proportion if there is a long running transaction. If optimal is specified (recommended), the roll back segment will shrink back to its optimal size.
    Wraps occur when the transaction has to carry forward to the next rollback extent. Wraps occur normally and do not affect performance if the next extent is inactive and available.



    Raminder Singh

    Oracle Certified DBA: Oracle 8i, 9i


    Mail me at raminderahluwalia@rediffmail.com.

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