Sizing the Buffer Cache doc

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Sizing the Buffer Cache

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Objectives

After completing this lesson, you should be able to do the following:

• Employ the buffer cache sizing advisor

• Describe how the buffer cache is used by different

Oracle processes

• Create and manage different buffer caches

• Monitor the use of the buffer caches

• Identify and resolve buffer cache

performance problems

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Overview

Data files

Checkpoint Queue

.

LRU lists

DBWn

Server

DB_BLOCK_SIZE DB_CACHE_SIZE DB_KEEP_CACHE_SIZE DB_RECYCLE_CACHE_SIZE DB_BLOCK_CHECKSUM

.

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Buffer Cache Sizing Parameters

• The buffer cache can consist of independent

subcaches for buffer pools and for multiple block sizes.

• The DB_BLOCK_SIZE parameter determines the

primary block size, which is the block size used for the system tablespace and the primary buffer caches (recycle, keep, and default).

• The following parameters define the sizes of the

caches for buffers for the primary block size:

– DB_CACHE_SIZE

– DB_KEEP_CACHE_SIZE

– DB_RECYCLE_CACHE_SIZE

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Dynamic Buffer Cache Advisory Parameter

• The buffer cache advisory feature enables and

disables statistics gathering for predicting behavior with different cache sizes.

• DBAs can use the information provided by these

statistics to size the buffer cache optimally for a given workload.

• The buffer cache advisory is enabled by means of

the DB_CACHE_ADVICE initialization parameter:

– This parameter is dynamic and can be changed using ALTER SYSTEM .

– Three values are allowed: Off, On, and Ready.

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View to Support Buffer Cache Advisory

• Buffer cache advisory information is collected in

the v$db_cache_advice view.

• The view contains different rows that estimate the

number of physical reads for different caches.

• The rows also compute a physical read factor,

which is the ratio of the number of estimated reads

to the number of actual reads.

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Using the v$db_cache_advice View

SQL> SELECT size_for_estimate "Cache Size (MB)",

2 buffers_for_estimate "Buffers",

3 estd_physical_read_factor AS

4 "Estd Phys Read Factor",

4 estd_physical_reads "Estd Phys Reads"

10 WHERE name = 'db_block_size')

11 AND advice_status = 'ON';

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Using the Buffer Cache Advisory

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.

LRU lists

.

Managing the Database Buffer Cache

4

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.

Managing the Database Buffer Cache

Server

Data files

LRU lists

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Tuning Goals and Techniques

– The v$db_cache_advice view

– Bypass the cache for sorting and parallel reads

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v$sysstat

Diagnostic Tools

SGA LRU list Checkpoint queue Buffer cache Keep buffer pool Recycle buffer pool v$buffer_pool

v$cache v$buffer_pool_statistics

Statspack Report v$db_cache_advice

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Performance Manager

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SQL> SELECT name, value

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More Buffer Cache Performance Indicators

From v$system_event:

SQL> SELECT event, total_waits

2 FROM v$system_event

3 WHERE event in

4 ('free buffer waits',

5 'buffer busy waits');

EVENT TOTAL_WAITS

-free buffer waits 337

buffer busy waits 3466

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Measuring the Cache Hit Ratio

SQL> SELECT 1 - (phy.value - lob.value - dir.value)

2 / ses.value "CACHE HIT RATIO"

3 FROM v$sysstat ses, v$sysstat lob,

4 v$sysstat dir, v$sysstat phy

5 WHERE ses.name = 'session logical reads'

6 AND dir.name = 'physical reads direct'

7 AND lob.name = 'physical reads direct (lob)'

8 AND phy.name = 'physical reads';

Statistic Total Per Per Trans Logon Second - - - - physical reads 15,238 13.0 15,238.0 physical reads direct 863 0.7 863.0 Physical reads direct(lob) 0 0 0 session logical reads 119,376 101.8 119,376.0

From v$sysstat:

From the Statspack report:

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Guidelines for Using the Cache Hit Ratio

Hit ratio is affected by data access methods:

• Full table scans

• Data or application design

• Large table with random access

• Uneven distribution of cache hits

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Buffer Cache Hit Ratio Isn’t Everything

• A badly tuned database can still have a hit ratio

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Guidelines to Increase the Cache Size

Increase the cache size ratio under the following conditions:

• Any wait events have been tuned

• SQL statements have been tuned

• There is no undue page faulting

• The previous increase of the buffer cache

was effective

• Low cache hit ratio

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Using Multiple Buffer Pools

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Defining Multiple Buffer Pools

In the Oracle database:

• Individual pools have their own size defined by:

– DB_CACHE_SIZE

– DB_KEEP_CACHE_SIZE

– DB_RECYCLE_CACHE_SIZE

• These parameters are dynamic.

• Latches are automatically allocated by Oracle

RDBMS.

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Enabling Multiple Buffer Pools

CREATE INDEX cust_idx … STORAGE (BUFFER_POOL KEEP …);

ALTER TABLE customer STORAGE (BUFFER_POOL RECYCLE); ALTER INDEX cust_name_idx

STORAGE (BUFFER_POOL KEEP);

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KEEP Buffer Pool Guidelines

• Tuning goal: Keeping blocks in memory

• Size: Holds all or nearly all blocks of the segments

assigned to this pool

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RECYCLE Buffer Pool Guidelines

• Tuning goal: Eliminate blocks from memory when

transactions are completed

• Size: Holds only active blocks

• Tool: v$cache

SQL> SELECT owner#, name, count(*) blocks

2 FROM v$cache

3 GROUP BY owner#, name;

OWNER# NAME BLOCKS

-

5 CUSTOMER 147

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RECYCLE Buffer Pool Guidelines

Tool: v$sess_io

SQL> SELECT s.username, io.block_gets,

2 io.consistent_gets, io.physical_reads

3 FROM v$sess_io io, v$session s

4 WHERE io.sid = s.sid ;

USERNAME BLOCK_GETS CONSISTENT_GETS PHYSICAL_READS - - - -

HR 21874 2327 1344

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Calculating the Hit Ratio for Multiple Pools

SQL> SELECT name, 1 - (physical_reads /

2 (db_block_gets + consistent_gets)) "HIT_RATIO"

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Identifying Candidate Pool Segments

• Keep pool

– Blocks are accessed repeatedly.

– Segment size is less than 10% of the default buffer

pool size.

• Recycle pool

– Blocks are not reused outside of transaction.

– Segment size is more than twice the default buffer

pool size.

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Dictionary Views with Buffer

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Automatic Segment Space Management

• Manages free space automatically inside database

segments

• Tracks segment free/used space with bitmaps

instead of free lists

• Provides better space utilization, especially for the

objects with highly varying size rows

• Specified when creating a tablespace

• Supported by Oracle Enterprise Manager

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Auto-Management of Free Space

• Create an auto-managed tablespace:

• Create a table that uses auto-management of

EXTENT MANAGEMENT LOCAL

SEGMENT SPACE MANAGEMENT AUTO;

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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 can 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.

• Using Automatic Free Space Management

eliminates the need for free lists, thus reducing contention on the database.

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Diagnosing Free List Contention

v$waitstat columns: CLASS “segment

header”

COUNT TIME

SGA Data buffer cache

v$system_event columns:

EVENT “buffer busy waits” TOTAL_WAITS

FREELISTS

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SGA Data buffer cache

Resolving Free List Contention

Server process

Object ID

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Multiple DBWn Processes

Multiple DB Writer (DBWn) processes:

• Can be deployed with DB_WRITER_PROCESSES

DISK_ASYNCH_IO parameter.

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Multiple I/O Slaves

• Provide nonblocking asynchronous I/O requests

• Are typically not recommended if asynchronous

I/O is available

• Follow the naming convention ora_innn_SID

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Tuning DBWn I/O

Tune the DB Writer processes by looking at the value

of the FREE BUFFER WAITS event.

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Summary

In this lesson, you should have learned how to:

• Employ the buffer cache sizing advisor

• Describe how the buffer cache is used by different

Oracle processes

• Create and manage different buffer caches

• Monitor the use of the buffer caches

• Identify and resolve buffer cache

performance problems

Tiêu đề	Sizing the Buffer Cache
Trường học	Oracle Corporation
Chuyên ngành	Database Management
Thể loại	essay
Năm xuất bản	2002

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Số trang	37
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