第四章、 buffer cache 调整
1、buffer cache 功能
buffer cache： 用于存放从datafile 里读出的数据块的镜像，并共享这些数据块，采用LRU算法
buffer cache 数据块状态：
pending：数据块正在使用，状态未决
dirty： 被修改过的块，还未写入到datafile
free：
已经从cache里写入到datafile的块，可以被覆盖
LRU LIST :链接从datafile里读出的块的头部信息，一般用于free 状态的块
Checkpoint list (Dirty
list):链接dirty block ，等待dbwr 写入到datafile
db_cache_size 指定默认池大小，一般数据块都放在默认池中，默认池管理采用LRU
db_keep_cache_size ：指定keep
池的大小，用于存放经常访问的小的对象的数据块。（比如索引块（常驻cache））
db_recycle_cache_size： 指定recycle
池的大小，用于存放偶尔访问对象的数据（比如偶尔做全表扫描的表的数据块）
2、设置buffer cache 的大小
对于sga 动态管理：
Granule 是内存分配的最小unit
Granule =4m sga<1g
Granule =16m sga>1g
----通过v$buffer_pool查看buffer cache
select NAME,BLOCK_SIZE,RESIZE_STATE,CURRENT_SIZE,BUFFERS from
v$buffer_pool
NAME BLOCK_SIZE RESIZE_STA
CURRENT_SIZE BUFFERS
--------------------------------------------------
---------- ---------- ------------ ----------
DEFAULT
8192 STATIC 156 19461
16384 STATIC 12
756
通过oracle advisory 调整buffer cache 的大小
15:01:33 SYS@ test1>show parameter db_cache
NAME TYPE
VALUE
------------------------------------ -----------
------------------------------
db_cache_advice string
ON
---默认oracle 会通过顾问来产生一个建议的大小
Buffer Pool Advisory DB/Inst: TEST1/test1 End Snap: 12
-> Only rows
with estimated physical reads >0 are displayed
-> ordered by Pool,
Block Size, Buffers For Estimate
Est
Phys Estimated Est
Size for Size Buffers
Read Phys Reads Est Phys % dbtime
P Est (M) Factr (thousands)
Factr (thousands) Read Time for Rds
--- -------- ----- ------------
------ -------------- ------------ --------
D 16 .1 2
1.4 6 28 32.9
D 32 .2 4
1.1 5 22 25.9
D 48 .3 6
1.1 5 21 24.7
D 64 .4 8
1.0 5 20 23.5
D 80 .5 10
D 96 .6 12
D 112 .7 14
D 128 .8 16
D 144 .9 18
D 160 1.0 20
D 164 1.0 20
size for est： buffer cache 期望的大小
size factr： 期望尺寸和实际尺寸的比率
buffers：
缓存的个数（数据块），以千为单位
est phys read factr：产生物理读得因子
从以上可以看出，buffer cache 在64m 是比较合适的，在16m ，产生物理读得可能性增加了40%
15:05:05 SYS@ test1>SELECT
size_for_estimate,buffers_for_estimate,estd_physical_read_factor,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‘;
SIZE_FOR_ESTIMATE BUFFERS_FOR_ESTIMATE ESTD_PHYSICAL_READ_FACTOR
ESTD_PHYSICAL_READS
----------------- --------------------
------------------------- -------------------
12 1497
2.1167 9421
24 2994
1.2083 5378
36
4491 1.0028 4463
48
5988 1 4451 ;当前比较合适的尺寸
60
7485 1 4451
72 8982 1 4451
84 10479 1 4451
96 11976 1
4451
108 13473 1
120 14970 1
132 16467
1 4451
144 17964
156 19461
168
20958 1 4451
180
22455 1 4451
192
23952 1 4451
204 25449 1 4451
216 26946
228 28443
240
29940 1 4451
20 rows selected.
3、buffer cache 调优目标
在oltp的库中，server 进程可以在buffer cache
找到所需要的块，提高块的命中率
1）通过oracle 顾问设置一个合适的尺寸，并提高块的命中率
2）设置不同的cache ：default 、keep 、recycle
3）将表缓存到cache
4）对于排序和并行读，跳过buffer cache
-------buffer cache 命中率
physical reads：物理读，直接从磁盘读取数据块到buffer cache
physical reads direct：
物理直接读，从磁盘读取数据块到pga ，跳过sga（一般比如用于排序操作）
physical reads direct (lob)：
对大对象的直接读
session logical reads：会话读取的所有数据块次数，包括物理读和一致性读等
13:56:17 SYS@ test1>SELECT ROUND(1 - ((physical.value - direct.value -
lobs.value) / logical.value),3) *100||‘%‘
13:56:19 2 "Buffer Cache Hit
Ratio"
13:56:27 3 FROM v$sysstat physical,
13:56:38 4 v$sysstat
direct,
13:56:47 5 v$sysstat lobs,
13:56:54 6 v$sysstat
logical
13:57:02 7 WHERE physical.name = ‘physical reads‘
13:57:13 8
AND direct.name = ‘physical reads direct‘
13:57:22 9 AND lobs.name =
‘physical reads direct (lob)‘
13:57:32 10 AND logical.name = ‘session
logical reads‘;
Buffer Cache Hit
Ratio
-----------------------------------------
99.5%
Elapsed: 00:00:00.04
13:57:41 SYS@ test1>
--对于OLTP的库，应该稳定在95% 以上。
4、通过建立不同的cache ，合理的使用buffer cache 空间，提高buffer cache的命中率
15:34:31 SYS@
test1>alter system set db_keep_cache_size=12m;
System altered.
Elapsed: 00:00:00.18
15:34:39 SYS@ test1>alter system set
db_recycle_cache_size=12m;
Elapsed: 00:00:00.09
15:34:53 SYS@ test1>show parameter cache
db_cache_size big integer 32M
db_keep_cache_size
big integer 12M
db_recycle_cache_size big
integer 12M
object_cache_max_size_percent integer
10
object_cache_optimal_size integer
102400
session_cached_cursors integer 20
案例：
使用keep 和recycle 池
---默认所有对象的块都会放入到default cache
15:37:24 SCOTT@ test1>select segment_name,segment_type,bytes/1024
,buffer_pool from user_segments
15:37:34 2 where segment_name in
(‘EMP‘,‘PK_EMP‘);
SEGMENT_NAME SEGMENT_TYPE BYTES/1024
BUFFER_
------------------------------ ------------------ ----------
-------
EMP TABLE 64
PK_EMP INDEX 64
Elapsed: 00:00:00.16
15:37:38 SCOTT@ test1>
----把经常调用的小的对象的块放入到keep （比如索引）
15:39:08 SCOTT@ test1>alter index pk_emp storage (buffer_pool keep);
Index altered.
Elapsed: 00:00:00.21
----对于偶然做全表扫描的表的数据块可以放入到recycle
15:39:34 SCOTT@ test1>alter table emp storage(buffer_pool recycle);
Table altered.
Elapsed: 00:00:00.07
15:40:06 SCOTT@ test1>select
segment_name,segment_type,bytes/1024 ,buffer_pool from user_segments
15:40:16
2 where segment_name in (‘EMP‘,‘PK_EMP‘);
RECYCLE
PK_EMP INDEX 64 KEEP
Elapsed: 00:00:00.03
15:40:23 SCOTT@ test1>
15:50:01 SCOTT@ test1>select ename,sal,deptno from emp where
deptno=10;
ENAME SAL DEPTNO
---------- ---------- ----------
CLARK
2450 10
KING 5000 10
MILLER
1300 10
----查看用户会话读取的数据块情况
15:49:19 SYS@ test1>select
s.username,io.block_gets,io.consistent_gets,io.physical_reads
15:49:33 2
from v$sess_io io,v$session s
15:49:37 3 where io.sid=s.sid and
s.username is not null;
USERNAME BLOCK_GETS CONSISTENT_GETS
PHYSICAL_READS
------------------------------ ---------- ---------------
--------------
SYS 9 899
61
SYS 0 16
1
Elapsed: 00:00:00.01
15:49:48 SYS@ test1>/
SCOTT 0 99
Elapsed: 00:00:00.02
block_gets ：在做dml 操作时读取的数据块
consistent_gets: SELECT
时读取的数据块
physical_reads :物理读
---KEEP 和 rcycle 池使用
14:20:27 SCOTT@ test1>alter index pk_emp storage(buffer_pool keep);
15:03:39 SCOTT@ test1>alter table emp storage(buffer_pool recycle);
14:36:09 SCOTT@ test1>select * from emp;
14:36:26 SYS@ test1>/
SCOTT 0 39
SYS 171 8717
549
14:40:07 SCOTT@ test1>update emp set sal=sal+1000 where deptno=20;
5 rows updated.
-----当update
操作时，由于emp表的块都放在recycle池中，select查询完成，这些数据块被消除；所以此次update操作，产生了物理读。
14:36:32
SYS@ test1>/
SCOTT 6 2999
94
Elapsed: 00:00:00.00
14:40:22 SCOTT@ test1>select * from emp1;
14:40:25 SYS@ test1>/
SCOTT 6 3069
101
14:41:16 SCOTT@ test1>update emp1 set sal=sal+1000 where deptno=20;
8 rows updated.
------emp表数据块在default 池，所以这些数据块被共享在buffer cache 里，在做update
时候，不会产生物理读。
14:41:22 SYS@ test1>/
SCOTT 14 3082
14:41:46 SYS@ test1>
15:07:51 SCOTT@
test1>analyze table emp estimate statistics;
Table analyzed.
Elapsed: 00:00:00.44
15:11:34 SCOTT@ test1>select * from emp where
ename=‘SCOTT‘;
EMPNO ENAME JOB MGR HIREDATE SAL COMM
DEPTNO
---------- ---------- --------- ---------- --------- ----------
---------- ----------
7788 SCOTT ANALYST 7566 19-APR-87
9000 20
Execution
Plan
----------------------------------------------------------
Plan hash
value: 3220259315
---------------------------------------------------------------------------------------------
|
Id | Operation | Name | Rows | Bytes | Cost (%CPU)|
Time
0 | SELECT STATEMENT | | 1 | 32 | 2 (0)|
00:00:01 |
| 1 | TABLE ACCESS BY INDEX ROWID| EMP | 1 |
32 | 2 (0)| 00:00:01 |
|* 2 | INDEX RANGE SCAN |
EMP_ENAME_IND | 1 | | 1 (0)| 00:00:01
Predicate Information (identified by operation
id):
---------------------------------------------------
2 - access("ENAME"=‘SCOTT‘)
Statistics
recursive calls
0 db block gets
3 consistent
gets
0 physical reads
0 redo size
828
bytes sent via SQL*Net to client
384 bytes received via SQL*Net from
client
2 SQL*Net roundtrips to/from client
0 sorts
(memory)
0 sorts (disk)
1 rows processed
15:11:43 SCOTT@ test1>
15:10:46 SYS@ test1>select
name,PHYSICAL_READS,DB_BLOCK_GETS,CONSISTENT_GETS from
v$buffer_pool_statistics;
NAME PHYSICAL_READS DB_BLOCK_GETS
CONSISTENT_GETS
-------------------- -------------- -------------
---------------
KEEP 2 0
6
RECYCLE 5 0
20
DEFAULT 4401 4271
85050
DEFAULT 0 0 0
5、关于buffer cache的等待事件
测试案例：
15:06:01 SCOTT@ test1>alter session enable parallel dml;
Session altered.
15:06:04 SCOTT@ test1> begin
15:06:18 2
for i in 1..500000 loop
15:06:18 3 execute immediate ‘insert /*+
parallel (4) */ into t1 values (‘||i||‘)‘;
15:06:18 4 end
loop;
15:06:18 5 end;
15:06:18 6 /
16:00:20 SYS@ test1>select event ,total_waits from
v$system_event
16:00:39 2 where event in (‘free buffer waits‘,‘buffer
busy waits‘);
EVENT
TOTAL_WAITS
----------------------------------------------------------------
-----------
buffer busy waits
16:01:06 SYS@ test1>
1 select name ,PARAMETER1,PARAMETER2 ,PARAMETER3 from v$event_name
2*
where name like ‘buffer busy%‘
NAME PARAMETER1 PARAMETER2
PARAMETER3
-------------------- -------------------- --------------------
--------------------
buffer busy waits file# block#
class#
buffer busy group# obj#
block#
------通过：
PARAMETER1： 处于等待事件中块所在的file#
PARAMETER2： 处于等待事件中块所在的
这样就可以通过两个参数来判断，这些块的类型（段头块、data 块、索引块、undo 块）
free buffer waits： server 进程从datafile 读取block 到buffer cache时，一般因为在buffer
cache没有空闲的buffer
解决： 增加buffer cache大小，或者让dbwr 写脏块释放空间
buffer busy
waits： 一般多个server 进程同时访问一个块
1）段头部：在做insert 时，因为free list 列表而引起的等待
解决：1）增加free list
2）将段空间管理采用自动方式（位图）
2）普通数据块：多个进程访问一个块
解决： 1）用小的块
2）增大pctfree
3）如果是索引块，索引是通过序列生成的主键，采用反向索引
3）undo 块：
多个server进程同时访问undo
解决： 1）对事务采用分批提交
2）增加rollback段或采用undo 表空间
---查询busy block 数据块类型及段
16:00:20 SYS@ test1> select ‘Segment Header‘
class, a.segment_type, a.segment_name, a.partition_name from dba_segments a,
v$session_wait b
where a.header_file = b.p1 and a.header_block = b.p2 and
b.event = ‘buffer busy waits‘
union
select ‘Freelist Groups‘ class,
a.segment_type, a.segment_name, a.partition_name from dba_segments a,
where b.p2 between a.header_block + 1 and (a.header_block +
a.freelist_groups) and a.header_file = b.p1 and a.freelist_groups > 1 and
select a.segment_type || ‘ block‘
class, a.segment_type, a.segment_name, a.partition_name from dba_extents a,
where b.p2 between a.block_id and a.block_id + a.blocks - 1
and a.file_id = b.p1 and b.event = ‘buffer busy waits‘
and not exists
(select 1 from dba_segments where header_file = b.p1 and header_block =
b.p2);
buffer busy waits：
#!/bin/bash
export ORACLE_SID=test1
count=0
while [ $count -lt 1000 ]
do
sqlplus ‘sys/oracle as sysdba‘<<EOF
update tb1 set object_id=1000 ;
rollback;
EOF
count=`expr $count + 1`
done
[oracle@rh6 ~]$ cat 4.sh
update tb1 set object_name=‘aaa‘ ;
[oracle@rh6 ~]$
13:35:30 SYS@ test1 > select
size_for_estimate,SIZE_FACTOR,buffers_for_estimate,estd_physical_read_factor,estd_physical_reads
13:36:11
2 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‘;
SIZE_FOR_ESTIMATE SIZE_FACTOR BUFFERS_FOR_ESTIMATE ESTD_PHYSICAL_READ_FACTOR
----------------- ----------- --------------------
4 .25
496 1.0955 71824
8 .5 992 1.0481
68718
12 .75 1488
1.0229 67070
16 1 1984
1 65565
20 1.25
2480 .9727 63772
24 1.5 2976 .9531
62492
28 1.75 3472
.9377 61483
32 2 3968
.9187 60235
8 rows selected.
13:36:12 SYS@ test1 >/
496 1.0963 79584
8 .5 992 1.05
76223
1.0239 74327
1 72591
2480 .9719 70552
24 1.5 2976 .9508
69023
.9335 67764
.9129 66267
13:36:49 SYS@ test1 >select event ,total_waits
from v$system_event
13:37:04 2 where event in (‘free buffer waits‘,‘buffer
free buffer waits
3623
4
13:37:10 SYS@ test1 >show parameter cache
client_result_cache_lag big
integer 3000
client_result_cache_size big integer
db_16k_cache_size big integer 12M
db_2k_cache_size
big integer 0
db_32k_cache_size big
integer 0
db_4k_cache_size big integer
db_8k_cache_size big integer 0
db_cache_advice
string ON
db_cache_size big
integer 16M
db_flash_cache_file
string
db_flash_cache_size big integer
db_keep_cache_size big integer
16M
db_recycle_cache_size big integer
result_cache_max_result integer
5
result_cache_max_size big integer 768K
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