Buffer Cache（缓冲区缓存）篇：keep缓冲区池（保留池）

Buffer  Cache可以有三个池

默认缓冲区池

keep缓冲区池

recycling缓冲区池

--保留池和回收池可以独立于sga中的其他缓存分配内存。创建表的时候可以在storage子句中使用buffer_pool_keep和buffer_pool_recyle子句来指定表的数据块将要驻留的池 。

keep缓冲区池

当数据读取到kepp缓冲区池后，会一直保留在内存中，不会被刷出内存，比如整天使用频繁的表，有利的做法是讲其放在keep缓冲池中，以减少IO数量。

keep池的大小由db_keep_cache_size决定，默认值为0

示例：

Setp1 设置db_keep_cache_size大小

SQL> alter system set db_keep_cache_size=32M scope=both;

System altered.

SQL>  show parameter keep

NAME                                 TYPE        VALUE
------------------------------------ ----------- ------------------------------
buffer_pool_keep                     string
control_file_record_keep_time        integer     7
db_keep_cache_size                  big integer 32M

/*--注buffer_pool_keep，8i参数，意义和db_keep_cache_size一样

增大db_keep_cache_size会相应减小db_cache_size，可以通过下面方式查看db_cache_size

SQL> select * from v$sgainfo;
Fixed SGA Size                              2217992 No
Redo Buffers                                2396160 No
Buffer Cache Size                         255852544 Yes
Shared Pool Size                          230686720 Yes
Large Pool Size                             4194304 Yes
Java Pool Size                              4194304 Yes
Streams Pool Size                                 0 Yes
Shared IO Pool Size                               0 Yes
Granule Size                                4194304 No
Maximum SGA Size                          839282688 No
Startup overhead in Shared Pool            67108864 No
Free SGA Memory Available                 339738624

或

SELECT x.ksppinm NAME,y.ksppstvl VALUE, x.ksppdesc describ
FROM SYS.x$ksppi x, SYS.x$ksppcv y
WHERE x.indx = y.indx AND x.ksppinm LIKE '%__db_cache_size%';

SQL> SELECT x.ksppinm NAME,y.ksppstvl VALUE, x.ksppdesc describ
  2  FROM SYS.x$ksppi x, SYS.x$ksppcv y
  3  WHERE x.indx = y.indx AND x.ksppinm LIKE '%__db_cache_size%';

NAME                           VALUE                          DESCRIB
------------------------------ ------------------------------ --------------------------------------------------------------------------------
__db_cache_size                255852544                      Actual size of DEFAULT buffer pool for standard block size buffers

*/

Setp2 将表keep到keep池中

方式1：

SQL> create table test_keep1 (id int,name varchar2(32));

Table created.

SQL>  alter table test_keep1 storage(buffer_pool keep);

Table altered.

方式2：

SQL> create table test_keep2 (id int,name varchar2(32)) storage(buffer_pool keep);

Table created.

Step3 查看放入keep池中的对象

SQL> select segment_name from dba_segments where BUFFER_POOL = 'KEEP';
TEST_KEEP1
TEST_KEEP2

Setp4 将表加载到keep池中

SQL> select * from test_keep1;

Setp5 检测IO情况

SET AUTOTRACE ON STATISTICS

select * from test_keep1;

Setp6 取消keep

alter table test_keep1 storage(buffer_pool default);

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