Estimating the size of the sbspace and metadata area

The first chunk of an sbspace must have a metadata area. When you add smart large objects, the database server adds more control information to this metadata area.

If you add a chunk to the sbspace after the initial allocation, you can take one of the following actions for metadata space:
  • Allocate another metadata area on the new chunk by default.

    This action provides the following advantages:

    • It is easier because the database server automatically calculates and allocates a new metadata area on the added chunk based on the average smart large object size
    • Distributes I/O operations on the metadata area across multiple disks
  • Use the existing metadata area

    If you specify the onspaces -U option, the database server does not allocate metadata space in the new chunk. Instead it must use a metadata area in one of the other chunks.

In addition, the database server reserves 40 percent of the user area to be used in case the metadata area runs out of space. Therefore, if the allocated metadata becomes full, the database server starts using this reserved space in the user area for additional control information.

You can let the database server calculate the size of the metadata area for you on the initial chunk and on each added chunks. However, you might want to specify the size of the metadata area explicitly, to ensure that the sbspace does not run out of metadata space and the 40 percent reserve area. You can use one of the following methods to explicitly specify the amount of metadata space to allocate:
  • Specify the AVG_LO_SIZE tag on the onspaces -Df option.

    The database server uses this value to calculate the size of the metadata area to allocate when the -Ms option is not specified. If you do not specify AVG_LO_SIZE, the database server uses the default value of 8 kilobytes to calculate the size of the metadata area.

  • Specify the metadata area size in the -Ms option of the onspaces utility.

    Use the procedure that Sizing the metadata area manually for a new chunk describes to estimate a value to specify in the onspaces -Ms option.


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