VLC - AG(AE) AP

TUNING

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VLC - AG(AE) AP

Tuning

• When is tuning necessary?

– Only if you feel that application is not running fast enough

• What is to be tuned?

– Oracle database

– Application

– Operating system

– Network

VLC - AG(AE) AP

Tuning Goals

• To optimize the performance of database

• To make database available to users without making them wait for resources

• To perform maintenance operations without interrupting users

VLC - AG(AE) AP

Tuning Parameters

• Response time

• Database availability

• Database hit percentages

• Memory utilization

VLC - AG(AE) AP

Tuning Steps

• Tune the design

• Tune the application

• Tune memory

• Tune IO

• Tune contention

• Tune operating system

VLC - AG(AE) AP

Tuning Considerations

• Different for

– OLTP databases

– DSS databases

– Hybrid databases

• Our database

– Hybrid database

– Data entry and Report generation done simultaneously

VLC - AG(AE) AP

Hybrid Databases

• Number of rollback segments for data entry

• Some large rollback segments for report generation

• Balanced db_block_buffers and log_buffers

• Medium size for db_block_size

VLC - AG(AE) AP

Tuning Hybrid Databases

• Tuning application is important in hybrid databases

• Indexing plays an important role

• Schedule batch processing if it is heavy on resources

• Use less number of bind variables in reports

• Optimize queries using hints

VLC - AG(AE) AP

Views, Utilities and Tools Used For Tuning

• Dynamic troubleshooting/performance and dictionary views

– V$XXXX dynamic troubleshooting and performance views

– DBA_XXX dictionary views

• Utlbstat.sql and utlestat.sql

• Alert logs and traces

• nitialization parameters

VLC - AG(AE) AP

Tuning Statistics To Be Gathered

• Library cache statistics

• System statistics

• Wait event statistics

• Latch statistics

• Rollback contention statistics

• Buffer busy wait statistics

• Dictionary cache statistics

• IO statistics per data file/table space

• Period of measurement

VLC - AG(AE) AP

Alert Log Files

• The lert log file consists of a chronological log of messages and errors

• Check the alert log file to:

– Detect internal errors and block corruption errors

– Monitor database operations

– View the non-default initialization parameters

• Remove or trim the alert log file regularly after checking

VLC - AG(AE) AP

Background Processes Trace Files

• The Oracle server dumps information about errors detected by any background process in trace files

• Oracle support uses these trace files to diagnose and trouble shoot problems

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User Trace Files

• Server process tracing is enabled or disabled at the session or instance level by:

– The ALTER_SESSION command

– The SET_SQL_TRACE_IN_SESSION procedure

– The initialization parameter SQL_TRACE

• A user trace file contain statistics for traced SQL statements for that session

• A user trace file is used for SQL tuning

• The Oracle database creates user trace files on per server process basis

VLC - AG(AE) AP

Tuning Memory

• The amount of memory occupied by Oracle is called SGA

• SGA contains

– DB Block Buffers

– Redo Log Buffers

– Shared Pool

o Library Cache

o Data Dictionary Cache

o UGA

– Large Pool

VLC - AG(AE) AP

Tuning Memory

• In hybrid databases like VLC, memory plays an important role

• Ideally DB_BLOCK_BUFFERS and LOG_BUFFERS to be in 80-20 ratio

• SHARED_POOL_SIZE also plays an important parameter in tuning memory

VLC - AG(AE) AP

Tuning Library Cache

• Library Cache tuned

– If the pins to misses ratio is more than 1%

– If the get hit ratio of name space is less than 90%

• V$Views

– V$LIBRARYCACHE

– V$SQLAREA

– V$SQLTEXT

– V$DB_OBJECT_CACHE

VLC - AG(AE) AP

Tuning Library Cache

• To find hit ratio of library cache

– Select namespace, gethitratio from v$librarycache

• To find reloads of SQL statements

– Select sql_text, users_executing, executions, loads from v$sqlarea

• To find library cache reloads

– Select sum(pins) Executions, sum(reloads) Misses, sum(reloads)/sum(pins) Hitratio from v$librarycache

VLC - AG(AE) AP

Tuning Library Cache

• Keep often used objects

• Avoid parsing

• Avoid large anonymous PL/SQL Blocks

– Select sql_text from v$sqlarea where command type=47;

• Reserve un-fragmentable memory in shared pool

– SHARED_POOL_RESERVED_SIZE

– SHARED_POOL_MIN_ALLOC

VLC - AG(AE) AP

Tuning Data Dictionary Cache

• Keep the ratio of the getmisses to gets less than 15%

– Select parameter, gets, getmisses from v$rowcache

• Increase shared pool size if the above ratio is more than 15%

VLC - AG(AE) AP

Tuning DB Block Buffer Cache

• DB hit ratio should always be more than 90%

• To find DB Cache hit ratio

– Select 1-(p.value)/(d.value+c.value) from v$sysstat p, v$sysstat d, v$sysstat c where p.name = ‘physical reads and d.name = ‘db block gets’ and c.name = ‘consistent gets’;

VLC - AG(AE) AP

Tuning Buffer Cache

• If DB hit ratio gets below 90%

– Increase buffer cache size

– Use multiple buffer pools

– Cache tables

– Bypass the buffer cache for sorting and parallel reads

VLC - AG(AE) AP

Tuning Buffer Pool

• To use multiple pool buffers set

– DB_BLOCK_BUFFERS

– DB_BLOCK_LRU_LATCHES

– BUFFER_POOL_KEEP

– BUFFER_POOL_RECYCLE

• To enable table caching

– Alter table with cache clause

– Use cache hint in query

VLC - AG(AE) AP

Tuning Redo Log Buffer

• Following parameters effect log buffer– LOG_BUFFER

– LOG_CHECK_POINT_INTERVAL

– LOG_CHECK_POINT_TIMEOUT

• Check for– Redo buffer space event

– Redo buffer allocation retries

– Redo log space requests

– Logfile switch (check point incomplete)

– Logfile switch (archive needed)

VLC - AG(AE) AP

Tuning Check Point

• Each checkpoint cause IO operations

• Frequent checkpoints reduces runtime performance

• Parameters

– FAST_START_IO_TARGET

– LOG_CHECK_POINT_INTERVAL

– LOG_CHECK_POINT_TIMEOUT

– DB_BLOCK_MAX_DIRTY_TARGET

– LOG_CHECK_POINT_TO_ALERT

VLC - AG(AE) AP

Tuning Large Pool

• Used for oracle maintenance operations

• UGA in MTS mode

• Parallel query

• IO-server process

• Tuning parameters

– LARGE_POOL_SIZE

– DBWR_IO_SLAVES

VLC - AG(AE) AP

Latches

• Contention areas that the DBA can tune:– Redo allocation latch

– Redo copy latch

– LRU latch

• Latch types – Willing to wait

o Gets, misses, sleeps

– Immediate

o Immediate gets and immediate waits

VLC - AG(AE) AP

LRU Latches

• LRU latches regulate the least recently used (LRU) lists used by the buffer cache

• By default, the Oracle server set the number of LRU latches to one-half the number of CPUs, with a minimum of one

• Each latch controls a minimum of 50 buffers

VLC - AG(AE) AP

LRU Latch Tuning Goals

• Ensure there are sufficient number of LRU latches for the data buffer cache so that contention between server processes is minimized

• Balance the number of latches with the number of CPUs

• Set one DBWn process for each latch

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Resolving LRU Latch Contention

• If the hit percentage for the LRU latch is less than 99%

– Increase the number of LRU latches by setting the parameter DB_BLOCK_LRU_LATCHES

– The maximum number of latches is the lower of:

o Number of CPUs*2*3

o Number of buffers/50

VLC - AG(AE) AP

Free Lists

• A free list for an object maintains a list of blocks that are available for inserts

• The number of free lists for an object cannot be set dynamically

• Single CPU systems do not benefit greatly from multiple free lists

• The tuning goal is to ensure that an object has sufficient free lists to minimize contention

VLC - AG(AE) AP

Resolving Free List Contention

• Query the V$SESSION_WAIT view

• Identify the object and get free lists for the segment from DBA_SEGMENTS

• Re-create the object in question

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Tuning IO-contentions

• Keep data files and log files on separate disks

• Stripe table data

• Reduce Disk IO

• Evaluate the use of RAW devices

• To know IO of files

– Select d.name, f.phyrds, f.phyrds from v$datafile d, v$filestat where d.file# = f.file#

VLC - AG(AE) AP

Oracle File Striping

• Operating system striping

– Use operating system striping software or RAID

– Decide on the right stripe size

• Manual striping

– Use the create table or alter table ALLOCATE command

– Is worth while with parallel query usage

VLC - AG(AE) AP

Tuning Data Files

• Keep data files and log files on separate disks to reduce IO contention

• Specify the initialization parameter DB_FILE_MULTIBLOCK_READ_COUNT

– To determine the number of database blocks the server reads at once

– To influence the execution plan of the cost-based optimizer

VLC - AG(AE) AP

Tuning Log Files

• Size redo log files to minimize the contention

• Have enough groups to prevent waiting

• Store redo log files on separate faster devices

• Dynamic views

– V$LOG

– V$LOGFILE

VLC - AG(AE) AP

Tuning Archive Log Files

• Keep archive log files on separate IO faster devices

• Archive log operations parameters

– LOG_ARCHIVE_MAX_PROCESSES

– LOG_ARCHIVE_DEST

• Dynamic views

– V$ARCHIVE_DEST

– V$ARCHIVE_LOG

– V$ARCHIVE_PROCESSES

VLC - AG(AE) AP

Tuning Rollback Segments

• Transactions should never wait for access to rollback segments

• Rollback segments should not extend during normal running

• Users and utilities should try to use less rollback

• No transaction should ever run out of rollback space

• Readers should always see the read-consistent images they need

VLC - AG(AE) AP

Tuning Rollback Segments

• The ratio of the sum of waits to the sum of gets should be less than 1%

– Select sum(gets) ‘gets’, sum(waits) ‘waits’, sum(waits)*100/sum(gets) ‘ratio’ from v$rollstat;

• If the ratio is more than 1% then create more rollback segments

• The number of waits for any class should be less than 1% of the total number of requests

VLC - AG(AE) AP

Sizing Transaction Rollback Data

• Deletes are expensive

• Inserts use minimal rollback space

• Updates use rollback space depending on the number of columns

• Index maintenance adds rollback

VLC - AG(AE) AP

Possible Problems

• Transactions fails for lack of rollback space

• “Snapshot too old” error occurs if:

– The inserted transaction list in the block being queried has been reused, and the SCN in the block is newer than the SCN at the start of the query

– The transaction slot in the rollback segment header has been reused

– The undo data in the rollback segment has been overlaid after a commit

VLC - AG(AE) AP

Tablespace Usage

• Reserve the SYSTEM tablespace usage for data dictionary objects

• Create locally managed tablespaces to avoid space management issues

• Split tables and indexes into separate tablespaces

• Create separate rollback tablespaces

• Store very large database objects in their own tablespace

• Create one or more temporary tablespaces