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Thread: The relation between log_buffer and redo log file size

  1. #1
    Join Date
    Aug 2000
    Location
    Shanghai
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    433
    Hi , Gurus ,

    what's the relation between log_buffer and redo log file size .

    My DB log_buffer is around 10MB but redo log file size is only 1 MB . I am worried that which log_buffer flush , the redo log file is not able to hold it
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  2. #2
    Join Date
    Jul 2002
    Location
    Lincolnshire, IL
    Posts
    203
    Ya this is a critical situation. Generally you should have redo log file with size atleast equal to log buffer size or bigger.
    Plz correct me if I'm wrong...
    Sandy
    "Greatest Rewards come only with Greatest Commitments!"

  3. #3
    Join Date
    Jun 2000
    Location
    Madrid, Spain
    Posts
    7,447
    10mb log_buffer is pretty useless, log buffer get flushed

    when

    1/3 full
    commit
    1MB of redo
    every 3 seconds
    before dbwr writes


  4. #4
    Join Date
    Jul 2002
    Posts
    132
    Redo records are buffered in a circular fashion in the redo log buffer of the SGA and are written to one of the online redo log files by the Oracle background process Log Writer (LGWR). Whenever a transaction is committed, LGWR writes the transaction's redo records from the redo log buffer of the SGA to an online redo log file, and a system change number (SCN) is assigned to identify the redo records for each committed transaction.
    Redo records can also be written to an online redo log file before the corresponding transaction is committed. If the redo log buffer fills, or another transaction commits, LGWR flushes all of the redo log entries in the redo log buffer to an online redo log file, even though some redo records may not be committed. If necessary, Oracle can roll back these changes.

    This is the relation between log buffer and redo log and so this is not at all a critical state if ur database is configured so. Normally any buffer shud be good enough to hold transactions and then written physically to the redo log files.

  5. #5
    Join Date
    Jun 2002
    Posts
    65
    Hi Sonia
    I know LGWR writes redo log entries in a circular fashion to Redo Log files but wasnt aware that redo log entries stored in redo log buffer in circular fasion as well...if not maistaken thatswhat you stated in reply...Right??

  6. #6
    Join Date
    Apr 2001
    Location
    London
    Posts
    725
    Originally posted by Fiza15
    Hi Sonia
    I know LGWR writes redo log entries in a circular fashion to Redo Log files but wasnt aware that redo log entries stored in redo log buffer in circular fasion as well...if not maistaken thatswhat you stated in reply...Right??
    Yes that is what sonia stated and correctly so. This is not the same concept as the circular fashion of redo log files.

    Here is a page which will explain.
    http://www.ixora.com.au/tips/tuning/log_buffer_size.htm
    Once you have eliminated all of the impossible,
    whatever remains however improbable,
    must be true.

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