Automatic Segment Management

Posted: 九月 8, 2011 in oracle

In a locally managed tablespace, there are two methods that Oracle Database can use to manage segment space: automatic and manual. Manual segment space management uses linked lists called “freelists” to manage free space in the segment, while auto segment space management uses bitmaps. Automatic segment space management is the more efficient method, and is the default for all new permanent, locally managed tablespaces.

When you allocate a datafile for a locally managed tablespace, you should allow space for metadata used for space management (the extent bitmap or space header segment) which are part of user space. For example, if specify the UNIFORM clause in the extent management clause but you omit the SIZE parameter, then the default extent size is 1MB. In that case, the size specified for the datafile must be larger (at least one block plus space for the bitmap) than 1MB.

■If you set extent management to LOCAL UNIFORM, then youmust ensure that each extent contains at least 5 database blocks.
■ If you set extent management to LOCAL AUTOALLOCATE, and ifthe database block size is 16K or greater, then Oracle manages segment space by creating extents with a minimum size of 5blocks rounded up to 64K.

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