Block Utilization Parameter


Block space utilization parameters can be used to control the space usage in index and data segments.

INITRANS

INITRANS specifies the initial number of transaction slots created in index or database blocks. The transaction slots are used to store the information about the transactions actively making changes to the block at a point in time. Any transaction uses only one transaction slot in a data block regardless of the number of rows or index entries being made by the transaction.

INITRANS defaults to 1 for data segments' database blocks, and 2 for index segments. This guarantees a minimum level of concurrency. If necessary, additional transaction slots can be allocated for the free space in the data block to allow for more concurrent transactions to modify data in the block.

MAXTRANS

MAXTRANS specifies the maximum number of transaction slots created in an index data block or a database data block. The default setting for MAXTRANS is 255, meaning that 255 concurrent transactions can be allowed for each given data or index data block. If this parameter is set, it restricts the use of space for transactions slots, therefore guaranteeing that there is sufficient space in the block for use by row and index data.

PCTFREE

PCTFREE, or percent free, for a data segment specifies the percentage of space in each data block that is reserved for any growth that results from updates to rows within the block. The default, for dictionary-managed tablespaces, is 10%. This parameter, if set, in a locally managed tablespace is ignored.

PCTFREE is calculated as a percentage of the available data space or block space remaining after deducting header space from the total block size.

PCTUSED

PCTUSED, or percent used, in a data segment represents the MINIMUM percentage of used space that Oracle tries to maintain for each block of the table's segment. A data block is placed back onto the free list when the used space falls below the PCTUSED threshold.

A free list is a list of blocks that are candidates for future inserts. Any given segment is created with one free list, although a segment can be created with more by setting the FREELISTS parameter of the storage clause.

FREELISTS are covered in detail in the Oracle 9i Performance Tuning exam.


The default setting for PCTUSED is 40% and, like PCTFREE, is ignored if set in a locally managed tablespace.

PCTUSED is calculated as a percentage of the available data space or block space remaining after deducting header space from the total block size.



    Oracle 9i Fundamentals I Exam Cram 2
    Oracle 9i Fundamentals I Exam Cram 2
    ISBN: 0789732653
    EAN: 2147483647
    Year: 2004
    Pages: 244
    Authors: April Wells

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