Thursday, April 19, 2007

Oracle9i DBA -- Database: Fundamentals I (20)



  • Query the data dictionary view DBA_FREE_SPACE to show how much free space is available in the database.
  • DBA_FREE_SPACE identifies the location and amount of free space by tablespace name, file ID, starting block ID, bytes, and blocks.
  • Query the data dictionary view DBA_SEGMENTS to display how much space is already used. And the location of all the tables and indexes by one users.
  • LGWR performs sequential writes from the redo log buffer cache to the redo log file under the following situations:

    1. When a transaction commits

    2. When the redo log buffer cache is one-third full

    3. When there is more than a megabyte of changes records in the redo log buffer cache

    4. Before DBWn writes modified blocks in the database buffer cache to the data files

    5. Every 3 seconds.

    Because the redo is needed for recovery, LGWR confirms the commit only after the redo is written to disk.

    LGWR can also call on DBWn to write to the data files.

    Note: DBWn does not write to the online redo logs.

  • There is a one-to-one correspondence between a user and server process, is called a dedicated server connection. When using a shared server configuration, it is possible for multiple user processes to share server processes.
  • 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. The 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.

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