BUFFER CACHE之调整buffer cache的大小

Buffer Cache存放真正数据的缓冲区，shared Pool里面存放的是sql指令(LC中一次编译，多次运行，加快处理性能，cache hit ratio要高)，而buffer cache里面存放真正的查询结果。Buffer Cache：由彼此独立的三个子cache（subcaches，也叫主buffer cache：keep，recycle，default）组成支持多种数据块的多缓冲池。注意system表空间只能用主数据块

Step1: 查看各个组件size（看buffer cache Method No.1）.

SQL> show parameter size

NAME                                 TYPE        VALUE

------------------------------------ ----------- ------------

bitmap_merge_area_size               integer     1048576

create_bitmap_area_size              integer     8388608

db_16k_cache_size                    big integer 0

db_2k_cache_size                     big integer 0

db_32k_cache_size                    big integer 0

db_4k_cache_size                     big integer 0

db_8k_cache_size                     big integer 0

db_block_size                      integer     8192

db_cache_size                      big integer 0

db_keep_cache_size                   big integer 0

db_recovery_file_dest_size           big integer 2G

NAME                                 TYPE        VALUE

------------------------------------ ----------- -----------

db_recycle_cache_size                big integer 0

global_context_pool_size             string

hash_area_size                       integer     131072

java_max_sessionspace_size           integer     0

java_pool_size                       big integer 0

large_pool_size                      big integer 0

max_dump_file_size                   string      UNLIMITED

object_cache_max_size_percent        integer     10

object_cache_optimal_size            integer     102400

olap_page_pool_size                  big integer 0

parallel_execution_message_size      integer     2148

NAME                                 TYPE        VALUE

------------------------------------ ----------- ------------

sga_max_size                         big integer 160M

shared_pool_reserved_size            big integer 2936012

shared_pool_size                     big integer 56M

sort_area_retained_size              integer     0

sort_area_size                       integer     65536

streams_pool_size                    big integer 0

workarea_size_policy                 string      AUTO

发现db_cache_size的值还是0，这个与shared_pool_size的情况也类似,10g文档描述：

If SGA_TARGET is set: If the parameter is not specified, then the default is 0 (internally determined by the Oracle Database). If the parameter is specified, then the user-specified value indicates a minimum value for the memory pool.

If SGA_TARGET is not set, then the default is either 48 MB or 4MB * number of CPUs * granule size, whichever is greater. 这样只有找到参数文件查看buffer cache的大小。

Step2: 动态指定db_cache_size的大小.

SQL> alter system set db_cache_size=92M scope=both;

System altered.

SQL> commit;

Commit complete.

1.DB_CACHE_SIZE指定的是基于主块大小(primary block size)的default缓冲池(buffer pool)的大小

2.该参数至少是4M*CPU个数*grunule大小。

采集统计数据用来预测不同cache size下的性能，用视图v$DB_CACHE_ADVICE查看。

Method No. 2：

SQL> select component,current_size,user_specified_size,granule_size

2  from v$sga_dynamic_components;

COMPONENT                      CURRENT_SIZE USER_SPECIFIED_SIZE GRANULE_SIZE

------------------------------ ------------ ------------------- ------------

shared pool                        58720256            58720256      4194304

large pool                          4194304                   0      4194304

java pool                           4194304                   0      4194304

streams pool                              0                   0      4194304

DEFAULT buffer cache               96468992            96468992      4194304

KEEP buffer cache                         0                   0      4194304

RECYCLE buffer cache                      0                   0      4194304

DEFAULT 2K buffer cache                   0                   0      4194304

DEFAULT 4K buffer cache                   0                   0      4194304

DEFAULT 8K buffer cache                   0                   0      4194304

DEFAULT 16K buffer cache                  0                   0      4194304

COMPONENT                      CURRENT_SIZE USER_SPECIFIED_SIZE GRANULE_SIZE

------------------------------ ------------ ------------------- ------------

DEFAULT 32K buffer cache                  0                   0      4194304

ASM Buffer Cache                          0            96468992      4194304

13 rows selected.

 

Step3: 查看是否启用动态buffer cache advisory参数.

SQL> show parameter advice

NAME                                 TYPE        VALUE

------------------------------------ ----------- ------------------------------

db_cache_advice                      string      ON

SQL> show parameter statisti

NAME                                 TYPE        VALUE

------------------------------------ ----------- ------------------------------

statistics_level                     string      TYPICAL

timed_os_statistics                  integer     0

timed_statistics                     boolean     TRUE

1.如果statistics_level的值是typical或all，则db_cache_size为on

2.三个值：on、off、ready；

ready是advisory关闭，但是系统为其分配了内存，off->ready->on,正常开启顺序；

ready->off/off->on,报错ORA-4031(inability to allocate from the shared pool)！只有ready->on->off来关闭！

 

Step4: 查看v$db_cache_advice视图收集的buffer cache advisory信息.

SQL> select name,size_for_estimate,estd_physical_read_factor,estd_physical_reads

2  from v$db_cache_advice;

NAME                 SIZE_FOR_ESTIMATE ESTD_PHYSICAL_READ_FACTOR ESTD_PHYSICAL_READS

-------------------- ----------------- ------------------------- -------------------

DEFAULT                              8                    1.6735               11840

DEFAULT                             16                    1.4867               10518

DEFAULT                             24                    1.3121                9283

DEFAULT                             32                    1.1869                8397

DEFAULT                             40                    1.1047                7816

DEFAULT                             48                    1.0329                7307

DEFAULT                             56                         1                7075

DEFAULT                             64                         1                7075

DEFAULT                             72                         1                7075

DEFAULT                             80                         1                7075

DEFAULT                             88                         1                7075

NAME                 SIZE_FOR_ESTIMATE ESTD_PHYSICAL_READ_FACTOR ESTD_PHYSICAL_READS

-------------------- ----------------- ------------------------- -------------------

DEFAULT                             92                         1                7075

DEFAULT                             96                         1                7075

DEFAULT                            104                         1                7075

DEFAULT                            112                         1                7075

DEFAULT                            120                         1                7075

DEFAULT                            128                         1                7075

DEFAULT                            136                         1                7075

DEFAULT                            144                         1                7075

DEFAULT                            152                         1                7075

DEFAULT                            160                         1                7075

21 rows selected.

 

NOTE: 在factor或reads变化不大的情况下，无需增加buffer cache因为不会带来significant benefit,根据视图，我的db_cache_size调到32M就够用了。

ESTD_PHYSICAL_READ_FACTOR:Physical read factor for this cache size, which is the ratio of the number of estimated physical reads to the number of reads in the real cache. If there are no physical reads in the real cache, the value of this column is null.

因为我没有非标准块所以直接查询无妨，标准的查询语句应为：

select size_for_estimate,buffers_for_estimate,estd_physical_read_fact,estd_physical_reads

from v$db_cache_advice

where name='DEFAULT'

AND block_size=(select value from v$parameter where name='db_block_size')

AND advice_status='ON';