Oracle Concepts - Oracle8 and Oracle8i System Global Area Oracle Tips by Burleson Consulting

System Global Area

The global memory area, called the System or Shared Global Area (SGA) is an area of CPU memory that is reserved for Oracle use only. It contains buffers that are used to speed transaction throughput and help maintain the system integrity and consistency. No data is altered directly on the disk; it all passes through the SGA. The size and configuration of the SGA is defined by a file called the initialization file or INIT.ORA file, which can contain information on each type of buffer or shared pool area in the SGA.

SGA

Figure 6 shows the SGA for Oracle8 and Oracle8i. Notice the numerous areas contained in the SGA and remember that each of these areas may be further subdivided into other related areas. The V$SGASTAT view has 39 areas listed in both an Oracle8 and Oracle8i database without multi-threaded server or Oracle Parallel server present.

Figure 6: Oracle8 and Oracle8i SGAs

Virtually all of the areas in the SGA are configured either directly (SHARED_POOL_SIZE, DB_BLOCK_BUFFERS, LOG_BUFFERS, LARGE_POOL_SIZE, etc.) or indirectly using initialization parameters. Improperly sized SGA areas can cause large performance problems.

Modifying the INIT.ORA

The INIT.ORA (or initSID.ora) file sets the initialization parameters for an instance specified by the SID contained in the file name. To modify the parameters permanently you must alter them in the initialization file. As was shown previously there are numerous parameters which can be altered on the fly or on a per session basis using various forms of the ALTER command.

The values for your initialization parameters can be easily obtained by looking at the initialization file or by looking at the V$PARAMETER view. The SVRMGRL SHOW PARAMETERS command can also be used to show parameter values.

Any editor can be used to modify the initialization parameter file but it must be stored as a text file. Anytime a parameter is altered the effects of the change should be closely monitored. Invalid parameters will cause Oracle to not startup.

Allocating And Caching Memory

The name of the game in Oracle configuration is properly allocating and using memory. By using the initialization parameters we allocate memory for use by Oracle. We can determine how this memory is used by creating or altering objects using the CACHE/NOCACHE and the KEEP and RECYCLE POOL options. The DB Block Buffer area can be split into the DEFAULT, KEEP and RECYCLE pools which should be used to optimize how objects are cached.

In Oracle8 and Oracle8i the database block buffer has been split into three possible areas, the default, keep and recycle buffer pool areas. It is not required that these three pools be used, only one, the default pool configured with the DB_BLOCK_BUFFERS initialization parameter must be present, the others are ?sub? pools to this main pool. How are the various pools used?

Use of the Default Pool

If a table, index or cluster is created without specifying that the KEEP or RECYCLE pool be used for its data, then it is placed in the default pool when it is accessed. This is standard Oracle7 behavior and if no special action is taken to use the other pools then this is also standard Oracle8 and Oracle8I behavior. The initialization parameters DB_BLOCK_BUFFERS and DB_BLOCK_LRU_LATCHES must be set if multiple pools are to be used:

DB_BLOCK_BUFFERS = 2100
DB_BLOCK_LRU_LATCHES = 18

Use of The KEEP Pool

The KEEP database buffer pool is configured using the BUFFER_POOL_KEEP initialization parameter which looks like so:

BUFFER_POOL_KEEP = ?300,3?

The two specified parameters are the number of buffers from the default pool to assign to the keep pool and the number of LRU (least recently used) latches to assign to the keep pool. The minimum number of buffers assigned to the pool is 50 times the number of assigned latches. The keep pool, as its name implies, is used to store object data that shouldn?t be aged out of the buffer pool such as look up information and specific performance enhancing indexes. The objects are assigned to the keep pool through either their creation statement or by specifically assigning them to the pool using the ALTER command. Any blocks already in the default pool are not affected by the ALTER command, only subsequently accessed blocks.

The keep pool should be sized such that it can hold all the blocks from all of the tables created with the buffer pool set to KEEP.

Use of the RECYCLE Pool

The RECYCLE database buffer pool is configured using the BUFFER_POOL_RECYCLE initialization parameter which looks like so:

BUFFER_POOL_RECYCLE = ?900,6?

The two specified parameters are the number of buffers from the default pool to assign to the recycle pool and the number of LRU (least recently used) latches to assign to the keep pool. The minimum number of buffers assigned to the pool is 50 times the number of assigned latches. The recycle pool, as its name implies, is used to store object data that should be aged out of the buffer pool rapidly such as searchable LOB information. The objects are assigned to the recycle pool through either their creation statement or by specifically assigning them to the pool using the ALTER command. Any blocks already in the default pool are not affected by the ALTER command, only subsequently accessed blocks.

As long as the recycle pool shows low block contention it is sized correctly.

With the above setpoints for the default, keep and recycle pools the default pool would end up with 900 buffers and 6 lru latches.

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