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Differential Backup

  • January 3, 2011 at 6:18 am

    #231644

    I do have one question regarding to differential backup.

    Suppose i am taking Full backup on sunday, and monday i am adding few rows in particular table and taking differntial backup.

    But how does SQL Server know which rows have to add using Differentail Backup?

  • January 3, 2011 at 6:31 am

    #1268439

    SQL has a type of allocation page called the DCM, the differential change map. When you make a change to a table, the extent that the modified/new rows are in gets flagged in the appropriate DCM (the bit for that extent goes from 0 to 1).

    The differential backup processes all extents that are flagged 1 in the DCMs. It doesn't care about the rows, just the extents (64kb chunk). Full backups reset the DCM to all 0

    Gail Shaw
    Microsoft Certified Master: SQL Server, MVP, M.Sc (Comp Sci)
    SQL In The Wild: Discussions on DB performance with occasional diversions into recoverability

    We walk in the dark places no others will enter
    We stand on the bridge and no one may pass

  • January 3, 2011 at 6:34 am

    #1268444

    SQL Server tracks changed data through Differential bitmap pages. Here is more information on it.

    Pradeep Adiga
    Blog: sqldbadiaries.com
    Twitter: @pradeepadiga

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