第五章 SGA其他缓存区调整
一、redo log buffer
1、redo log 的功能
1）Sever 进程在buffer
cache修改数据块后，Oracle提倡‘先记后写’，对修改的数据块的改变生成log entries（日志条目),将日志条目按顺序写入log
buffer；而对于脏块
先链接到检查点队列，等待dbwr 进程写入到datafile
2） 进程会及时的将其按顺序写入到redo log
files。（日志条目的数据块大小一般和操作系统数据块大小一致，在Unix 和linux 下一般为512Bytes）
3）lgwr 在以下条件会写log
entries：
A)、commit
B)、log buffer 空间1/3
C)、日志到1M时
D)、每隔3S
E)、在dbwr 写脏块前
--redo 日志及时的写入到redo log files，可以在instance 或 media recover
时，通过redo log 来对数据库进行recovery
2、redo log buffer 大小
09:59:26 [email protected] test1>select name ,bytes/1024/1024 from v$sgainfo;
NAME
BYTES/1024/1024
-------------------------------- ---------------
Fixed SGA
Size 1.16233444
Redo Buffers
2.8359375
----由于redo log buffer的空间循环使用，可以不用设太大的尺寸，一般默认值即可。
3、监控log buffer
10:07:36 [email protected] test1>select sid,event,seconds_in_wait,state
10:07:59 2
from v$session_wait
10:08:05 3 where event=‘log buffer space‘;
no rows selected
------‘log buffer space’ 事件，指的是当server 进程写日志条目到log buffer
时，没有可用的空间，而导致等待
10:10:58 [email protected] test1>begin
10:11:28 2 for i in
1..1000000
10:11:38 3 loop
10:11:41 4 execute immediate ‘insert into
t1 values (:num)‘ using i;
10:11:46 5 end loop;
10:11:50 6
end;
10:11:51 7 /
PL/SQL procedure successfully completed.
10:13:10 [email protected] test1>r
1 select sid,event,seconds_in_wait,state
2 from v$session_wait
3* where event=‘log buffer space‘;
10:12:04
[email protected] test1>/
SID EVENT
SECONDS_IN_WAIT STATE
----------
---------------------------------------------------------------- ---------------
-------------------
20 log buffer space
9 WAITED KNOWN TIME
Elapsed: 00:00:00.01
10:12:24 [email protected] test1>/
SID EVENT
SECONDS_IN_WAIT STATE
----------
---------------------------------------------------------------- ---------------
-------------------
20 log buffer space
22 WAITED KNOWN TIME
Elapsed: 00:00:00.00
-----事务量增加，导致了“log buffer space” 的等待事件
10:17:55 [email protected] test1>select username,sid from v$session where username is
not null;
USERNAME SID
------------------------------
----------
SYS 39
SCOTT
28
Elapsed: 00:00:00.02
10:18:07 [email protected] test1>select SID,EVENT,TOTAL_WAITS
WAIT_CLASS from v$session_event
10:18:29 2 where sid in(39,28) and event
in (‘log file sync‘,‘log file parallel write‘);
SID EVENT
WAIT_CLASS
----------
----------------------------------------------------------------
----------
39 log file sync
5
Elapsed: 00:00:00.03
10:18:30 [email protected] test1>
log file sync等待事件
当用户发出提交或回滚语句时会触发LGWR将重做记录写入联机日志文件，这种触发LGWR的方式叫做同步写（sync
writes）触发，
而其他剩下的触发LGWR的方式叫做后台写（background writes）。log file sync等待事件只与sync
writes有关，
而log file parallel write等待事件只与background writes有关。
-----调整（增加日志组或日志组的大小，或者增加log_buffer 的大小）
10:35:42 [email protected] test1>alter system set log_buffer =10240000
scope=spfile;
System altered.
10:38:53 [email protected] test1>
begin
10:39:03 [email protected] test1>for i in
1..1000000
loop
10:39:03 3 execute immediate ‘insert into t1
values (:num)‘ using i;
10:39:03 5 end loop;
10:39:03 6
end;
10:39:03 7 /
PL/SQL procedure successfully completed.
Elapsed: 00:01:39.52
10:40:44 [email protected] test1>
10:38:17 [email protected] test1>select sid,event,seconds_in_wait,state
10:39:14 2
from v$session_wait
10:39:14 3 where event=‘log buffer space‘;
no rows selected
Elapsed: 00:00:00.11
10:39:15 [email protected] test1>/
no rows selected
Elapsed: 00:00:00.04
10:39:42 [email protected] test1>/
no rows selected
Elapsed: 00:00:00.05
10:39:51 [email protected] test1>/
no rows selected
Elapsed: 00:00:00.09
10:43:18 [email protected] test1>select SID,EVENT,TOTAL_WAITS WAIT_CLASS from
v$session_event
10:43:19 2 where sid in(39,22) and event in (‘log file
sync‘,‘log file parallel write‘);
no rows selected
-------调整了redo buffer 空间后，log buffer space 等待事件被消除
-------查看日志条目的信息
10:20:30 [email protected] test1>select name,value from v$sysstat
10:20:51 2
where name in (‘redo buffer allocation retries‘,‘redo entries‘);
NAME
VALUE
----------------------------------------------------------------
----------
redo entries
1125660
redo buffer allocation retries
9
Elapsed: 00:00:00.01
------redo entries 生成的redo 日志条目的个数
------redo buffer allocation retries
server 进程在写入日志条目时，没有足够的空间，重试的次数
--一般要求，重试次数应该为0，两者相比的比率小于1%
4、减少redo 的生成
1)、在做sql load 时采用direct 导入和nologging。
2)、在执行sql 语句时，使用nologging（但dml
操作无效）
------dml
0:32:36 [email protected] test1>update emp1 nologging set sal=sal+2000 where
deptno=10;
12 rows updated.
Elapsed: 00:00:00.01
Execution
Plan
----------------------------------------------------------
Plan hash
value: 2879231406
-----------------------------------
| Id | Operation | Name
|
-----------------------------------
| 0 | UPDATE STATEMENT |
|
| 1 | UPDATE | EMP1 |
|* 2 | TABLE ACCESS FULL| EMP1
|
-----------------------------------
Predicate Information (identified by operation
id):
---------------------------------------------------
2 - filter("DEPTNO"=10)
Note
-----
- rule based optimizer used (consider using cbo)
Statistics
----------------------------------------------------------
1
recursive calls
6 db block gets
23 consistent
gets
0 physical reads
3036 redo size
681
bytes sent via SQL*Net to client
589 bytes received via SQL*Net from
client
4 SQL*Net roundtrips to/from client
1 sorts
(memory)
0 sorts (disk)
12 rows processed
10:32:44 [email protected] test1>
------在nologging 下仍有日志生成
------通过视图监控
10:36:58 [email protected] test1>select a.name,b.value from v$statname a,v$mystat
b
10:37:05 2 where a.STATISTIC#=b.STATISTIC# and a.name like ‘%redo
size%‘;
NAME
VALUE
----------------------------------------------------------------
----------
redo size
269785556
10:37:11 [email protected] test1>update emp1 nologging set sal=sal+2000 where
deptno=10;
12 rows updated.
10:37:31 [email protected] test1>select a.name,b.value from v$statname a,v$mystat
b
10:37:45 2 where a.STATISTIC#=b.STATISTIC# and a.name like ‘%redo
size%‘;
NAME
VALUE
----------------------------------------------------------------
----------
redo size
269790652
------DDL
-------使用nologging 创建索引
10:40:32 [email protected] test1>create index t1_id_ind on t1(id) nologging;
Index created.
Elapsed: 00:00:04.53
10:41:12 [email protected] test1>select a.name,b.value from
v$statname a,v$mystat b
10:41:25 2 where a.STATISTIC#=b.STATISTIC# and
a.name like ‘%redo size%‘;
NAME
VALUE
----------------------------------------------------------------
----------
redo size
269910748
Elapsed: 00:00:00.00
10:41:33 [email protected] test1>
------正常创建索引
10:20:30 [email protected] test1>select name,value from v$sysstat
10:20:51 2
where name in (‘redo buffer allocation retries‘,‘redo entries‘);
NAME
VALUE
----------------------------------------------------------------
----------
redo entries
1125660
redo buffer allocation retries
9
Elapsed: 00:00:00.01
10:20:56 [email protected] test1>create table t1 as select *
from scott.t1;
Table created.
Elapsed: 00:00:02.48
10:43:04 [email protected] test1>select a.name,b.value from
v$statname a,v$mystat b
10:43:18 2 where a.STATISTIC#=b.STATISTIC# and
a.name like ‘%redo size%‘;
NAME
VALUE
----------------------------------------------------------------
----------
redo size
13852204
Elapsed: 00:00:00.02
10:43:29 [email protected] test1>create index t1_id_index on
t1(id);
Index created.
Elapsed: 00:00:17.44
10:44:08 [email protected] test1>select a.name,b.value from
v$statname a,v$mystat b
10:44:20 2 where a.STATISTIC#=b.STATISTIC# and
a.name like ‘%redo size%‘;
NAME
VALUE
----------------------------------------------------------------
----------
redo size
34085060
Elapsed: 00:00:00.01
10:44:24 [email protected] test1>
15:09:25 [email protected] test1 > select sid,event,seconds_in_wait,state
15:09:58
2 from v$session_wait
15:09:58 3 where event like ‘log%‘;
no rows selected
Elapsed: 00:00:00.00
15:09:59 [email protected] test1 >/
SID EVENT
SECONDS_IN_WAIT STATE
----------
---------------------------------------------------------------- ---------------
-------------------
11 log file parallel write
0 WAITING
Elapsed: 00:00:00.00
15:10:02 [email protected] test1 >/
no rows selected
Elapsed: 00:00:00.00
15:10:03 [email protected] test1 >/
SID EVENT
SECONDS_IN_WAIT STATE
----------
---------------------------------------------------------------- ---------------
-------------------
11 log file parallel write
0 WAITING
36 log buffer space
1 WAITING
37 log buffer
space 1
WAITING
Elapsed: 00:00:00.00
15:10:04 [email protected] test1 >
解决方法：
可以调整log buffer
空间大小（不推荐），主要是调整lgwr的写出的速度
1、对于大的事务量，尽量采用分批提交
2、将redo 日志存放到磁盘I/O
最快的磁盘上（比如 采用RAID10方式）
3、增加日志组，增加日志组的大小
4、如果是归档模式，可再启用多个归档进程
二、java
pool
在oracle 10g 采用内存自动管理，可动态分配java pool的大小
三、启动异步读写
异步： 当dbwr 写时，后台进程可以继续运行
同步： 当dbwr 写时，后台进程必须处于等待状态（串行方式）
10:44:24 [email protected]
test1>show parameter syn
NAME TYPE
VALUE
------------------------------------ -----------
------------------------------
disk_asynch_io boolean
TRUE
tape_asynch_io boolean TRUE
10:56:49 [email protected]
test1>
------可以启用多个dbwr 进程（dbw0-dbw9）
10:57:37 [email protected] test1>show parameter
writer
NAME TYPE
VALUE
------------------------------------ -----------
------------------------------
db_writer_processes integer
1
10:57:56 [email protected] test1>
----但和cpu个数有关，如果是多个cpu ，可以启动多个dbwr
db_writer_processes integer 1
10:57:56 [email protected]
test1>show parameter cpu
NAME TYPE
VALUE
------------------------------------ -----------
------------------------------
cpu_count integer
1