8-Storage Structure

Pctused storage structure tathagata bhattacharjee

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Unformatted text preview: tacharjee Block Space Utilization Parameters 1. INITRANS 2. MAXTRANS 3. PCTFREE 4. PCTUSED Storage Structure Tathagata Bhattacharjee • INITRANS and MAXTRANS specify the initial and maximum number of transaction slots that are created in an index or a data block. • The transaction slots are used to store information about transactions that are making changes to the block at a point in time. • A transaction uses only one transaction slot, even if it is changing more than one row or index entry Storage Structure Tathagata Bhattacharjee • INITRANS, which defaults to 1 for data segment and 2 for index segment, guarantees a minimum level of concurrency . • E.g., if set to 3, INITRANS ensures that at least three transactions can concurrently make changes to the block. If necessary, additional transaction slots can be allocated from the free space in the block to permit more concurrent transactions to modify rows in the blocks Storage Structure Tathagata Bhattacharjee • MAXTRANS, which has a default value of 255, sets the limit for the number of concurrent transactions that can make changes to a data or an index block. • When set, this value restricts use of space for transaction slots and therefore guarantees that there is sufficient space in the block for use by row or index data Storage Structure Tathagata Bhattacharjee • PCTFREE for a data segment specifies the percentage of space in each data block reserved for growth resulting from updates to rows in the block. The default value is 10% Storage Structure Tathagata Bhattacharjee • PCTUSED for a data segment represents the minimum percentage of used space that the Oracle server tries to maintain for each data block of the table. • A block is put back on the free list when its used space falls below PCTUSED. • A free list of a segment is a list of blocks that are candidates for accommodating future inserts • A segment, by default, is created with one free list. Segments can be created with a higher number of free lists by setting the FREELISTS parameter of the storage clause. The default value is 40% Storage Structure Tathagata Bhattacharjee – Automatic Segment-Space Management – Manual Configuration • Two methods are available for managing data blocks: Data Block Management Storage Structure – Better concurrency handling Tathagata Bhattacharjee • All objects and especially objects with greatly varying row sizes utilize space more effectively – Better space utilization • P...
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