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Inside Oracle9i Tablespace Management

2013-11-13 15:38:04  來源: Oracle 

  by Donald K Burleson
  
  Over the past few releases Oracle has been automating and improving the internal administration of tables and indexes It has gradually recognized the benefits of bitmap data structures in all areas of the database Recently Oracle has introduced two new tablespace parameters that automate storage management functions:
  
  LMT (Locally Managed Tablespaces) — The LMT tablespace is implemented by adding EXTENT MANAGEMENT LOCAL clause to the tablespace definition LMT tablespaces automate extent management and remove the ability to specify the NEXT storage parameter The only exception is when NEXT is used with MINEXTENTS at table creation time
  ASSM (Automatic Segment Space Management) — The ASSM tablespace is implemented by adding the SEGMENT SPACE MANAGEMENT AUTO clause to the tablespace definition ASSM tablespaces automate FREELIST management and remove the ability to specify PCTUSED FREELISTS and FREELIST GROUPS storage parameters You cannot use ASSM unless you also use LMTs on a tablespace
  It is important to note that LMT and ASSM are optional and are used in the same instance with traditional tablespaces Remember LMT and ASSM are implemented at the tablespace level and each instance can have LMT LMT and ASSM tablespaces or traditional tablespaces
  
  Before we discuss the differences between bitmap FREELISTS and traditional FREELIST management lets examine how bitmap FREELISTS are implemented We begin by creating a tablespace with the segment space management auto parameter Note that ASSM is only valid for locallymanaged tablespaces with extent management local syntax
  
  create tablespace
  asm_test
  datafile
  c:\oracle\oradata\diogenes\asm_testdbf
  size
  m
  EXTENT MANAGEMENT LOCAL
  SEGMENT SPACE MANAGEMENT AUTO
  ;
  
  
  Once a table or index is allocated in this tablespace the values for PCTUSED for individual objects will be ignored and Oraclei will automatically manage the FREELISTS for the tables and indexes inside the tablespace For objects created in this tablespace the NEXT extent clause is now obsolete because of the locallymanaged tablespace (except when a table is created with MINEXTENTS and NEXT) The INITIAL parameter is still required because Oracle cannot know in advance the size of the initial table load When using Automatic Space Management the minimum value for INITIAL is three blocks
  
  There is some debate about whether a onesizefitsall approach is best for Oracle In large databases individual object settings can make a huge difference in both performance and storage As we may know the setting for PCTUSED governs FREELIST relinking If we want high disk space usage we set PCTUSED to a value xzignthly greater than avg_row_len Conversely if we want fast INSERT performance we set PCTUSED to a low value ensuring that all free blocks are nearly empty providing lots of block space for INSERT operations
  
  The Issue of PCTFREE
  
  
  As a quick review the PCTFREE parameter is used to specify the amount of free space on a data block to reserve for future row expansion If PCTFREE is set improperly SQL update statements can cause a huge amount of row fragmentation and chaining
  
  The setting for PCTFREE is especially important where a row is initially stored small and expanded at a later time In such systems it is not uncommon to set PCTFREE equal to telling Oracle to reserve percent of the data block space for subsequent row expansion
  
  Fortunately Oraclei does not allow you to specify the value for PCTFREE if you are using Automatic Space Management Row chaining is a serious problem for the DBA and it appears that Automatic Space Management is still appropriate for tables for which you need to reserve space for large row expansions with PCTFREE
  
  The Issue of PCTUSED
  
  
  As we know improper settings for PCTUSED can cause huge degradation in the performance of SQL inserts If a data block is not largely empty excessive I/O will happen during SQL inserts because the reused Oracle data blocks will become full quickly Taken to the extreme improper settings for PCTUSED can create a situation in which the free space on the data block is smaller than the average row length for the table In these cases Oracle will try five times to fetch a block from the FREELIST chain After five attempts Oracle will raise the highwater mark for the table and grab five fresh data block for the insert
  
  In Oraclei with Automatic Segment Management the PCTUSED parameter no longer governs the relink threshold for a table data block and we must rely on the judgment of Oracle to determine when a block is empty enough to be placed onto the FREELIST
  
  Unlike PCTFREE in which Oracle cannot tell in advance how much row expansion will occur Oraclei does have information about the right time to relink a data block Because Oracle knows the average row length for the table rows (dba_tablesavg_row_Len) Oracle should be able to adjust PCTUSED to ensure that the relinked data block will have room for new rows
  
  An Oracle Inconsistency
  
  
  While Oraclei ignores the PCTUSED FREELISTS and FREELIST GROUPS parameters with LMT and ASSM tablespaces Oracle does not give an error message when these ignored parameters are used in a table definition
  
  SQL> create table
   test_table
   (c number)
   tablespace
   asm_test
   pctfree pctused
   storage
   ( freelists next m ) ;
  
  Table created
  
  
  
  Most Oracle DBAs would assume that invalid parameters would be treated as they have been treated since Oracle and reported as an error
  
  SQL> create index
   test_type_idx
   on
   book(book_type)
   PCTUSED ;
  PCTUSED
  *
  ERROR at line :
  ORA: invalid CREATE INDEX option
  
  
  
  This could lead to confusion when a DBA believes that they are changing these values when in reality tablespaces with LMT or SAM ignore any specified values for PCTUSED NEXT and FREELISTS
  
  
  No More Buffer Busy Waits
  
  
  One huge benefit of Automatic Segment Management is the bitmap FREELISTS that are guaranteed to reduce buffer busy waits Lets take a close look at this feature
  
  Prior to Oraclei buffer busy waits were a major issue As a review a buffer busy wait occurs when a data block is inside the data buffer cache but it is unavailable because it is locked by another DML transaction A block was unavailable because another SQL insert statement needed to get a block on which to place its row Without multiple FREELISTS every Oracle table and index had a single data block at the head of the table to manage the free block for the object Whenever any SQL insert ran it had to go to this block and get a data block on which to place its row
  
  Obviously single FREELISTS cause a backup When multiple tasks wanted to insert into the same table they were forced to wait while Oracle assigned free blocks one at a time
  
  Oracles Automatic Segment Space Management feature claims to improve the performance of concurrent DML operations significantly since different parts of the bitmap can be used simultaneously eliminating serialization for free space lookups
  
  According to Oracle benchmarks using bitmap FREELISTS removes all segment header contention and allows for superfast concurrent insert operations (refer to figure )
  
  
  

From:http://tw.wingwit.com/Article/program/Oracle/201311/17040.html
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