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db_ultra_safe Tips

Oracle Tips by Burleson Consulting

January 2, 2015

Question:  I have seen a new 10g parameter called db_ultra_safe.  Anytime I gear something described as ultra safe, I worry that it is just marketing hype.

What does the db_ultra_safe parameter do, and do I want to implement db_ultra_safe?  My disks are mirrored and super-reliable with a mean time between failures of 30 years.

Answer:  The db_ultra_safe init.ora parameter is a meta parameter got Oracle's file corruption checking facility.  If you have "safe" disks (at least two years without a file corruption issue, you may not want to turn-on the overhead of file checking.

However, if you are using replication where redo logs and updates are transferred between systems (RAC, Data Guard, Streams standby database), the possibility of data corruption may increase slightly.

The db_ultra_safe parameter is a meta parameter, such that a single setting sets the values for multiple other parameters.

Oracle knows that with some additional overhead, you can add extra checking to ensure that your data files are not corrupted:

  • db_block_checking:  This db_block_checking checks data block integrity before writing the data block to disk.  In this involves checking the internal block headers and footer, plus internal row linkages.

  • db_block_checksum: In 10g, this will allow the DBWR (database writer) and sqlldr.exe program to do a checksum when writing data to disk.  In 11g, the checks are moved to foreground processes. to add less overhead.
  • db_lost_write_protect is introduced in 11g to turn-off file write checking overhead.  This is important on disk arrays with their own RAM that issue false "acks", acknowledging that data has been written, while it is actually still i  the disk array data buffer.

The db_ultra_safe parameter sets the default values for other parameters that control protection levels.

  • db_ultra_safe= off:  When any of db_block_checking, db_block_checksum, or db_lost_write_protect are explicitly set, no changes are made.
  •  db_ultra_safe=data_only:  This single setting for db_ultra_safe sets these three values:

    - db_block_checking = medium
    - db_lost_write_protect = typical
    - db_block_checksum = full.
  • db_ultra_safe=data_and_index

    - db_block_checking will be set to full.
    - db_lost_write_protect will be set to typical.
    - db_block_checksum will be set to full.

As we se, the db_ultra_safe set the overhead values for systems with unreliable disks.  Ships with mirrored disks may not need this feature.



 
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