Oracle9Oracle9ii Performance TuningPerformance Tuning

Chapter 2Chapter 2

Tuning the Buffer CacheTuning the Buffer Cache

Chapter 2: Tuning the Buffer Cache 2

Chapter ObjectivesChapter Objectives

• Understand how the buffer cache works

• Learn how to configure the buffer cache

• Learn how to configure a database with one or multiple database block sizes

• Dynamically allocate SGA memory

• Configure a buffer cache with multiple buffer pools

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Chapter Objectives Chapter Objectives (continued)(continued)

• Understand how automatic table caching works

• Learn how to use the new Buffer Cache Size Advice

• Diagnose buffer cache configuration

• Look inside the buffer cache

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Oracle ArchitectureOracle Architecture

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Buffer Cache OverviewBuffer Cache Overview

• The SGA is the memory structure in which Oracle caches data that is retrieved for:• Access

• Updates

• Submitted SQL statements

• Executed PL/SQL blocks

• Data dictionary definitions

• Other cache mechanisms

• The major purpose of the SGA is to enhance data retrieval by placing the most frequently used data in memory rather than retrieving it from disk

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Buffer Cache Overview Buffer Cache Overview (continued)(continued)

• The SGA consists of the following memory structures:• Buffer cache• Redo log buffers• Shared pool memory• Large pool• Java pool

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Buffer Cache Overview Buffer Cache Overview (continued)(continued)

• The buffer cache stores the most frequently accessed Oracle data blocks to reduce disk I/O• If data is cached, the Oracle server sends back the data

requested without returning to the data files (this is known as a cache hit)

• If data is not cached, the Oracle server fetches the data from data files in the database based on the execution plan (this is known as a cache miss)

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Buffer Cache Overview Buffer Cache Overview (continued)(continued)

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Buffer Cache Overview Buffer Cache Overview (continued)(continued)

• The types of buffers are:• Default

• Keep

• Recycle

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Configuring the Buffer CacheConfiguring the Buffer Cache• Initial configuration can be based on existing databases and types

of applications• The DB_BLOCK_BUFFERS parameter is deprecated in

Oracle9i but is still supported for backward compatibility• DB_BLOCK_BUFFERS cannot be allocated dynamically• Use DB_CACHE_SIZE to size the buffer• DB_CACHE_SIZE can be changed without shutting down the

database• The Buffer Cache Size Advice feature is enabled only if the

DB_CACHE_SIZE parameter is used• The SGA_MAX_SIZE parameter sets the maximum size the

SGA can grow to

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Database with Multiple Block SizesDatabase with Multiple Block Sizes• Buffers support tablespaces with

different block sizes than the one set with the DB_BLOCK_SIZE parameter

• Parameters:• DB_2K_CACHE_SIZE• DB_4K_CACHE_SIZE• DB_8K_CACHE_SIZE• DB_16K_CACHE_SIZE

• DB_32K_CACHE_SIZE

SQL> CREATE TABLESPACE TEST_DATA_16K 2 LOGGING 3 DATAFILE 4 'C:\ORACLE\ORADATA\SAM\TEST_DATA_16K.ORA' 5 SIZE 100M 6 BLOCKSIZE 16384 7 EXTENT MANAGEMENT LOCAL 8 /

SQL> SELECT TABLESPACE_NAME, BLOCK_SIZE 2 FROM DBA_TABLESPACES;

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Dynamic SGA AllocationDynamic SGA Allocation• Use SGA_MAX_SIZE if you need to increase the

SGA allocated memory• Use V$SGA_DYNAMIC_COMPONENTS to view

the size of the major memory structures in the SGA• When you are adjusting the size of any memory

structure in the SGA, it is decremented or incremented in granules• In Oracle9i, a granule is a memory unit

Chapter 2: Tuning the Buffer Cache 13

Dynamic SGA Allocation Dynamic SGA Allocation (continued)(continued)

Chapter 2: Tuning the Buffer Cache 14

Dynamic SGA Allocation Dynamic SGA Allocation (continued)(continued)

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Configuring Multiple Buffer PoolsConfiguring Multiple Buffer Pools• DB_KEEP_CACHE_SIZE configures memory allocation for the KEEP pool

in the buffer cache• DB_RECYCLE_CACHE_SIZE configures memory allocation for the

RECYCLE pool in the buffer cache• Total size of memory for buffers is the sum of the following parameter values:

• DB_CACHE_SIZE• DB_KEEP_CACHE_SIZE• DB_RECYCLE_CACHE_SIZE• DB_2K_CACHE_SIZE• DB_4K_CACHE_SIZE• DB_8K_CACHE_SIZE• DB_16K_CACHE_SIZE• DB_32K_CACHE_SIZE

SQL> SELECT SUM(VALUE)/(1024*1024) 2 FROM V$PARAMETER 3 WHERE NAME IN('db_cache_size','db_keep_cache_size', 4 'db_recycle_cache_size', 'db_2k_cache_size', 5 'db_4k_cache_size', 'db_8k_cache_size', 6 'db_16k_cahce_size', 'db_32_cache_size') 7 /

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Configuring Multiple Buffer Pools Configuring Multiple Buffer Pools (continued)(continued)

• Cache a table in a specific buffer:

SQL> ALTER TABLE DEPARTMENTS 2 STORAGE ( BUFFER_POOL KEEP) 3 /

SQL> SELECT /*+ CACHE(CATEGORIES) */ * FROM CATEGORIES /

Method 1

Method 2

SQL> ALTER TABLE DEPARTMENTS CACHE;

SQL> SELECT TABLE_NAME, CACHE, BUFFER_POOL 2 FROM USER_TABLES 3 ORDER BY TABLE_NAME 4 /

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Automatic Table CachingAutomatic Table Caching• Oracle9i Release 2 has introduced the automatic

caching of small tables based on the size of the table• Small table: a table is considered small if it is less than 20

blocks or 2% of the total cached blocks

• Medium table: a table is considered medium if it is more than 20 blocks and less than 10% of the total cached blocks

• Automatic table caching is based on these criteria, caching statistics, and table scan frequency

• These criteria do not apply to any table that has the CACHE option enabled

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Buffer Cache AdviceBuffer Cache Advice

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Buffer Cache Advice Buffer Cache Advice (continued)(continued)

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Buffer Cache Advice Buffer Cache Advice (continued)(continued)

SQL> SELECT DECODE(SIZE_FACTOR, 1, '==>', null) " ", SIZE_FOR_ESTIMATE CSIZE ,

TRUNC(SIZE_FACTOR*100)||'%' PERCENT, BUFFERS_FOR_ESTIMATE BUFFERS_EST,

ESTD_PHYSICAL_READ_FACTOR E_PHY_READ_FACTOR, ESTD_PHYSICAL_READS E_PHY_READS

FROM V$DB_CACHE_ADVICE WHERE NAME = 'DEFAULT' AND ADVICE_STATUS = 'ON' AND BLOCK_SIZE = (SELECT VALUE FROM V$PARAMETER WHERE NAME = 'db_block_size');

CACHE SIZE PERCENT BUFFERS_EST E_PHY_READ_FACTOR E_PHY_READS --- ---------- ------- -------------- ----------------- --------------- 4 33% 500 2.1007 6084420 8 66% 1000 1.3393 3879163 ==> 12 100% 1500 1 2896319 16 133% 2000 0.8335 2414111 20 166% 2500 0.7709 2232899 24 200% 3000 0.7646 2214471 28 233% 3500 0.755 2186828 32 266% 4000 0.7444 2156114 . . .

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Buffer Cache Advice Buffer Cache Advice (continued)(continued)

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Buffer Cache DiagnosisBuffer Cache Diagnosis• Use V$SYSSTAT and V$SESSTAT• Buffer cache hit ratio is a percentage of physical reads

over logical reads• Hit ratio = 1 - (physical reads/(block gets + consistent gets))/100• Physical reads is the total number of data blocks accessed from disk• Block gets is the total number of buffers that are obtained for update

(means the data block was read for update)• Consistent gets is the total number of buffers that are obtained in

consistent read (means that the data block that was accessed used the System Change Number (SCN) to determine that the data block being read did not change since the query was submitted)

SELECT ROUND( (1 - (PHY.VALUE/(CUR.VALUE + CON.VALUE)))*100, 1)||'%' ratio 2 FROM V$SYSSTAT PHY, V$SYSSTAT CUR, V$SYSSTAT CON 3 WHERE PHY.NAME = 'physical reads' 4 AND CUR.NAME = 'db block gets' 5 AND CON.NAME = 'consistent gets';

Chapter 2: Tuning the Buffer Cache 23

Buffer Cache Diagnosis Buffer Cache Diagnosis (continued)(continued)

• Buffer cache hit ratio threshold

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Buffer Cache Diagnosis Buffer Cache Diagnosis (continued)(continued)

• Hit ratio per session

SQL> SELECT PHY.SID, 2 S.USERNAME, 3 1 - (PHY.VALUE)/(CUR.VALUE + CON.VALUE) BUFFER_HITRATIO 4 FROM V$SESSTAT PHY, V$SESSTAT CUR, V$SESSTAT CON, 5 V$STATNAME S1, V$STATNAME S2, V$STATNAME S3, 6 V$SESSION S 7 WHERE S1.NAME = 'physical reads' 8 AND S2.NAME = 'db block gets' 9 AND S3.NAME = 'consistent gets' 10 AND PHY.STATISTIC# = S1.STATISTIC# 11 AND CUR.STATISTIC# = S2.STATISTIC# 12 AND CON.STATISTIC# = S3.STATISTIC# 13 AND CUR.VALUE <> 0 14 AND CON.VALUE <> 0 15 AND PHY.SID = CUR.SID 16 AND PHY.SID = CON.SID 17 AND PHY.SID = S.SID 18 /

Chapter 2: Tuning the Buffer Cache 25

Buffer Cache Diagnosis Buffer Cache Diagnosis (continued)(continued)

• Buffer pool statistics are shown using the performance dynamic view V$BUFFER_POOL_STATISTICS

• Block size buffer hit ratio

SQL> COLUMN RATIO HEADING "Buffer Cache Hitratio" FORMAT A30 SQL> COLUMN NAME HEADING 'Buffer Pool' FORMAT A15SQL> COLUMN BLOCK_SIZE HEADING 'Block SQL> SELECT Size' SELECT NAME, 2 BLOCK_SIZE, 3 ROUND( (1 - (PHYSICAL_READS/ 4 (DB_BLOCK_GETS + CONSISTENT_GETS)))*100) || '%' ratio 5 FROM V$BUFFER_POOL_STATISTICS 6 /

SQL> SELECT BLOCK_SIZE, 2 ROUND( (1 - AVG((PHYSICAL_READS/ 3 (DB_BLOCK_GETS + CONSISTENT_GETS))))*100) || '%' ratio 4 FROM V$BUFFER_POOL_STATISTICS 5 GROUP BY BLOCK_SIZE 6 /

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Inside the Buffer CacheInside the Buffer Cache• Use the V$BH view to:

• Query the view for a list of data objects residing in the buffers and the number of data blocks in use

• See how many blocks are modified (dirty) and how many are free or read

• Inspect blocks that have been rewritten and re-read in an Oracle Real Application Cluster configuration

• Find out more about which data files or tablespaces being accessed most often and how many buffers they use

Chapter 2: Tuning the Buffer Cache 27

Inside the Buffer Cache Inside the Buffer Cache (continued)(continued)

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SummarySummary• The buffer cache is a memory structure of the SGA• The buffer cache facilitates faster access to data, because when

data is retrieved from a disk, the cost to performance is high• The Least Recent Used algorithm ages out the least retrieved and

changed blocks of data from the buffer• The buffer cache consists of three major internal structures: the

Default buffer pool, Keep buffer pool, and Recycle buffer pool• The DB_CACHE_SIZE parameter configures the size of the

buffer cache