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Thread: SCN in the block ?

  1. #11
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    1. ckpt process does not write at commit
    2. lgwr writes at commit only to redologs that includes writing scn as well.
    3. at checkpoint datafileheaders/controlfiles are updated with scns
    4. certainly there is no block level scn Regards

  2. #12
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    Of course there is a SCN recorded in each and every block! And each and every block in datafile could have different SCN recorded in it. SCN in block marks the point in time when last changes have been written into that block.

    This is the basic principle that allow Oracle to generate a read consistent view for a query. If SCN in a block is greater than te one in which query was started Oracle knows this block was changed after the query has started, so it must obtain a before change immage of that block from rollback segment.
    Jurij Modic
    ASCII a stupid question, get a stupid ANSI
    24 hours in a day .... 24 beer in a case .... coincidence?

  3. #13
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    sm8728, quester,

    Oracle blocks DO contain the SCN. Read through this and you will know how and why it is there.

    http://www.ixora.com.au/notes/cache_header.htm

    Sanjay




    [Edited by SANJAY_G on 09-20-2002 at 12:59 AM]

  4. #14
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    SM....
    Sanjay is right...every block contains an SCN number.

    I am pasting the quote from oracle documetation, just go
    through this...
    --------------------

    Oracle8i Recovery Manager User's Guide and Reference
    Release 2 (8.1.6)

    How Incremental Backups Work

    Each data block in a datafile contains an SCN, which is the SCN at which the last change was made to the block. During an incremental backup,
    RMAN reads the SCN of each data block in the input file and compares it to the checkpoint SCN of the parent incremental backup. RMAN reads the
    entire file every time whether or not the blocks have been used.
    ------------------------------

    sanjy, u were talking about blocking the dump...i just tried
    that, but could not figure out to which path does it write to...can u just explain me abt the same...
    regards
    sonia

  5. #15
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    Originally posted by sonia
    sanjy, u were talking about blocking the dump...i just tried
    that, but could not figure out to which path does it write to...can u just explain me abt the same...
    Sonia, blocking the dump or dumping the block
    The trace file goes to user_dump_dest.

    Sanjay

    [Edited by SANJAY_G on 09-20-2002 at 05:25 AM]

  6. #16
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    when is the block scn written to at commit or at checkpoint which process writes this to the block . can u clarify pls .
    u mean to say that the scn no written to datafile header may be different from block scn . I am curous about these .

  7. #17
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    Dear sir

    I am also wondering why SCN number is to be written to
    data block in Datafile.

    Normally all header information is getting updated due to
    checkpoint process.
    Checkpoint does synchronization and DBWR will write to datafile header

    So Oracle Experts What is correct things
    ----------------------------------------------
    Viraj
    Oracle OCA

    A Wise Man Knows How much he doesn't know !!!

  8. #18
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    Originally posted by virajvk

    Dear sir
    I am also wondering why SCN number is to be written to
    data block in Datafile.
    Viraj
    Oracle OCA
    Read the complete thread properly.. Again!!!

    Originally posted by jmodic

    Of course there is a SCN recorded in each and every block! And each and every block in datafile could have different SCN recorded in it. SCN in block marks the point in time when last changes have been written into that block.

    This is the basic principle that allow Oracle to generate a read consistent view for a query. If SCN in a block is greater than te one in which query was started Oracle knows this block was changed after the query has started, so it must obtain a before change immage of that block from rollback segment.


    [Edited by Sameer on 09-20-2002 at 06:04 AM]

  9. #19
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    The most confusion comes from the acronym SCN, bacause it is a common acronym for many different things in Oracle. Among other things, it can mean:

    Sytem Commit Number
    System Checkpoint Number
    System Change Number

    In the context of this therad SCN means System Change Number, which can be think of as Oracle's internal clock mechanysm. When whatever change happens to Oracle database this counter is incremented, no matter if that change will later on be commited or not.
    Jurij Modic
    ASCII a stupid question, get a stupid ANSI
    24 hours in a day .... 24 beer in a case .... coincidence?

  10. #20
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    Originally posted by jmodic
    Of course there is a SCN recorded in each and every block! And each and every block in datafile could have different SCN recorded in it. SCN in block marks the point in time when last changes have been written into that block.

    This is the basic principle that allow Oracle to generate a read consistent view for a query. If SCN in a block is greater than te one in which query was started Oracle knows this block was changed after the query has started, so it must obtain a before change immage of that block from rollback segment.
    Thanks for sharing this information. This is really a good way of looking at it.

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