Storage Structure and Relationships参考.ppt

Storage Structure and Relationships Objectives After completing this lesson, you should be able to do the following: Describe the logical structure of the database List the segment types and their uses List the keywords that control block space usage Obtain information about storage structures from the data dictionary List the criteria for separating segments Overview Types of Segments Types of Segments Types of Segments Storage Clause Precedence Extent Allocation and Deallocation Allocated when the segment is: Created Extended Altered Deallocated when the segment is: Dropped Altered Truncated Used and Free Extents Database Block Minimum unit of I/O Consists of one or more OS blocks Set at tablespace creation DB_BLOCK_SIZE is the default block size Database Block Contents Block Space Utilization Parameters Data Block Management Two methods are available for managing data blocks: Automatic Segment-Space Management Manual Configuration Automatic Segment-SpaceManagement It is method of managing free space inside database segments Tracking in-segment free and used space is done using bitmaps as opposed to free lists This method provides: Ease of management Better space utilization Better performance for concurrent INSERT operations Restriction: Can not be used for tablespaces which will contain LOBs. Automatic Segment-SpaceManagement Bitmap segments contain a bitmap that describes the status of each block in the segment with respect to its available space. The map is contained in a separate set of blocks referred to as bitmapped blocks (BMBs). When inserting a new row, the server searches the map for a block with sufficient space. As the amount of space available in a block changes, its new state is reflected in the bitmap. Configuring Automatic Segment-Space Management Automatic segment-space management can be enabled at the tablespace level only, for locally managed tablespaces. After a tablespace is created, the specifications apply to all segments created in