Effect of db_block_size on RMAN Backups
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Thread: Effect of db_block_size on RMAN Backups

  1. #1
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    Effect of db_block_size on RMAN Backups

    Can anybody point me to a document that explains the effect a block size has on an RMAN backup? It seems as though my databases with a smaller db_block_size backup about 20% slower than those with a bigger db_block_size.
    Jeff Hunter
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  2. #2
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    Sorry..cannot point to any doc but are you taking a full level (0) backup with RMAN ? If so, then remember RMAN will ignore the unused blocks. Since chances of blocks being used are more if its smaller they may take more time.
    Vinit

  3. #3
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    Yes, it's a level 0 backup.
    Jeff Hunter
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  4. #4
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    Originally posted by vinit
    Since chances of blocks being used are more if its smaller
    How so? In theory, if I have 1M of data in a 2k db_block_size, I would be using 512 blocks. If that same data uses 16K db_block_size, I would only be using 64 blocks.

    However, since each database block is not 100% full with data, I am thinking that the 20% difference comes from having to read more overhead in the smaller blocks. I would like to find some documentation that supports or disproves my theory...
    Jeff Hunter
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  5. #5
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    Originally posted by marist89

    However, since each database block is not 100% full with data, I am thinking that the 20% difference comes from having to read more overhead in the smaller blocks. I would like to find some documentation that supports or disproves my theory...
    You are right about the block overhead and thats what I had pointed out. Smaller the block size, more blocks used and more time for backup. Secondly it doesn't matter if the block is 100% used or not. Even if the block is empty during the backup and was used by Oracle previously it will be backed up.
    Vinit

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