11g内存管理新特性的internal表现

11g中自动内存管理(Automatic Memory Management ,amm), 令dba在数据库内存配置的相关工作更加简单. AMM现在将SGA与PGA整合到一起管理,而您只需要设置memory_target参数即可限定Oracle将使用到的内存尺寸,Oracle将自动分配这些内存空间.

您一定很困惑Oracle在unix平台上是如何对共享的sga内存空间与私有的pga内存空间进行切换的?这意味着Oracle需要经常释放sga中的部分内存以便允许pga去使用它们.传统的sys V 使用的共享内存shm接口不具备如此的灵活性.我们来看看Oracle是如何做到的?

先来获取我们需要的11g实例共享内存id(shared memory id)

[oracle@rh2 ~]$ sysresv                // 该命令需要设置了正确的LD_LIBRARY_PATH
IPC Resources for ORACLE_SID “T11” :
Shared Memory:
ID              KEY
65537           0x95c84bb8
Semaphores:
ID              KEY
327681          0xdf521034
Oracle Instance alive for sid “T11”

试着找出对应的sys V共享内存段:

[oracle@rh2 ~]$ ipcs -m

—— Shared Memory Segments ——–
key        shmid      owner      perms      bytes      nattch     status
0x95c84bb8 65537      oracle    660        4096       0

对应的存在着共享内存段,但该段很小只有 4096 byte哦,既然Oracle不再把sga放到共享段中,那藏到哪里去了呢?

我们接下来检查Oracle实例进程的内存影射状况.

[oracle@rh2 ~]$  pmap `pgrep -f lgwr`|less
14889:   ora_lgwr_T11
0000000000400000 155016K r-x–  /usr/oracle/product/11g/db_1/bin/oracle
0000000009c62000  12404K rw—  /usr/oracle/product/11g/db_1/bin/oracle
000000000a87f000    732K rwx–    [ anon ]
0000000060000000      4K r–s-  /dev/shm/ora_T11_65537_0
0000000060001000  16380K rw-s-  /dev/shm/ora_T11_65537_0
0000000061000000  16384K rw-s-  /dev/shm/ora_T11_65537_1
0000000062000000  16384K rw-s-  /dev/shm/ora_T11_65537_2
0000000063000000  16384K rw-s-  /dev/shm/ora_T11_65537_3
0000000064000000  16384K rw-s-  /dev/shm/ora_T11_65537_4
0000000065000000  16384K rw-s-  /dev/shm/ora_T11_65537_5
0000000066000000  16384K rw-s-  /dev/shm/ora_T11_65537_6
0000000067000000  16384K rw-s-  /dev/shm/ora_T11_65537_7
0000000068000000  16384K rw-s-  /dev/shm/ora_T11_65537_8
0000000069000000  16384K rw-s-  /dev/shm/ora_T11_65537_9
000000006a000000  16384K rw-s-  /dev/shm/ora_T11_65537_10
000000006b000000  16384K rw-s-  /dev/shm/ora_T11_65537_11
000000006c000000  16384K rw-s-  /dev/shm/ora_T11_65537_12
000000006d000000  16384K rw-s-  /dev/shm/ora_T11_65537_13
000000006e000000  16384K rw-s-  /dev/shm/ora_T11_65537_14
000000006f000000  16384K rw-s-  /dev/shm/ora_T11_65537_15
0000000070000000  16384K rw-s-  /dev/shm/ora_T11_65537_16
0000000071000000  16384K rw-s-  /dev/shm/ora_T11_65537_17
0000000072000000  16384K rw-s-  /dev/shm/ora_T11_65537_18
0000000073000000  16384K rw-s-  /dev/shm/ora_T11_65537_19
0000000074000000  16384K rw-s-  /dev/shm/ora_T11_65537_20
0000000075000000  16384K rw-s-  /dev/shm/ora_T11_65537_21
0000000076000000  16384K rw-s-  /dev/shm/ora_T11_65537_22
0000000077000000  16384K rw-s-  /dev/shm/ora_T11_65537_23
0000000078000000  16384K rw-s-  /dev/shm/ora_T11_65537_24
0000000079000000  16384K rw-s-  /dev/shm/ora_T11_65537_25
000000007a000000  16384K rw-s-  /dev/shm/ora_T11_65537_26
000000007b000000  16384K rw-s-  /dev/shm/ora_T11_65537_27
000000007c000000  16384K rw-s-  /dev/shm/ora_T11_65537_28
000000007d000000  16384K rw-s-  /dev/shm/ora_T11_65537_29
000000007e000000  16384K rw-s-  /dev/shm/ora_T11_65537_30
000000007f000000  16384K rw-s-  /dev/shm/ora_T11_65537_31
0000000080000000  16384K rw-s-  /dev/shm/ora_T11_65537_32
0000000081000000  16384K rw-s-  /dev/shm/ora_T11_65537_33
0000000082000000  16384K rw-s-  /dev/shm/ora_T11_65537_34
0000000083000000  16384K rw-s-  /dev/shm/ora_T11_65537_35
0000000084000000  16384K rw-s-  /dev/shm/ora_T11_65537_36
0000000085000000  16384K rw-s-  /dev/shm/ora_T11_65537_37
0000000086000000  16384K rw-s-  /dev/shm/ora_T11_65537_38
0000000087000000  16384K rw-s-  /dev/shm/ora_T11_65537_39
0000000088000000  16384K rw-s-  /dev/shm/ora_T11_65537_40
0000000089000000  16384K rw-s-  /dev/shm/ora_T11_65537_41
000000008a000000  16384K rw-s-  /dev/shm/ora_T11_65537_42

............

0000003e79109000      4K rw—  /lib64/tls/librt-2.3.4.so
0000003e7910a000     64K rw—    [ anon ]
0000003e79600000     84K r-x–  /lib64/libnsl-2.3.4.so
0000003e79615000   1020K —–  /lib64/libnsl-2.3.4.so
0000003e79714000      4K r—-  /lib64/libnsl-2.3.4.so
0000003e79715000      4K rw—  /lib64/libnsl-2.3.4.so
0000003e79716000      8K rw—    [ anon ]
0000007fbfff3000     52K rwx–    [ stack ]
ffffffffff600000      4K r-x–    [ anon ]
total          2497724K

pmap工具诠释了进程相关共享内存的情况,可以看到许多个16MB的"文件"对应了Oracle服务进程的空间地址.这是linux上POSIX风格的共享内存管理模式,使用"文件"形式包含共享内存段.

借助于将sga分割成许多块,Oracle可以很容易地把sga部分内存返回给OS,而服务器进程即可以利用到这些内存.(当memory_max_target>1024时,颗粒为16MB,否则为4MB).

接下来我们测试下Oracle是如何释放部分sga内存的.

对比实例启动前后:

启动前:

[oracle@rh2 ~]$ ls -l /dev/shm
总用量 0

启动后:
[oracle@rh2 ~]$ ls -l /dev/shm
总用量 1373704
-rw-r—–  1 oracle oinstall 16777216  9月 27 18:59 ora_T11_327680_0
-rw-r—–  1 oracle oinstall 16777216  9月 27 18:59 ora_T11_327680_1
-rw-r—–  1 oracle oinstall        0  9月 27 18:59 ora_T11_327680_10
-rw-r—–  1 oracle oinstall 16777216  9月 27 18:59 ora_T11_327680_100
-rw-r—–  1 oracle oinstall 16777216  9月 27 18:59 ora_T11_327680_101
-rw-r—–  1 oracle oinstall 16777216  9月 27 18:59 ora_T11_327680_102
-rw-r—–  1 oracle oinstall 16777216  9月 27 18:59 ora_T11_327680_103
-rw-r—–  1 oracle oinstall 16777216  9月 27 18:59 ora_T11_327680_104
-rw-r—–  1 oracle oinstall 16777216  9月 27 18:59 ora_T11_327680_105
-rw-r—–  1 oracle oinstall 16777216  9月 27 18:59 ora_T11_327680_106
-rw-r—–  1 oracle oinstall 16777216  9月 27 18:59 ora_T11_327680_107
-rw-r—–  1 oracle oinstall 16777216  9月 27 18:59 ora_T11_327680_108
-rw-r—–  1 oracle oinstall 16777216  9月 27 18:59 ora_T11_327680_109
-rw-r—–  1 oracle oinstall        0  9月 27 18:59 ora_T11_327680_11
-rw-r—–  1 oracle oinstall 16777216  9月 27 18:59 ora_T11_327680_110
-rw-r—–  1 oracle oinstall 16777216  9月 27 18:59 ora_T11_327680_111
-rw-r—–  1 oracle oinstall 16777216  9月 27 18:59 ora_T11_327680_112
-rw-r—–  1 oracle oinstall 16777216  9月 27 18:59 ora_T11_327680_113
-rw-r—–  1 oracle oinstall 16777216  9月 27 18:59 ora_T11_327680_114
-rw-r—–  1 oracle oinstall 16777216  9月 27 18:59 ora_T11_327680_115
-rw-r—–  1 oracle oinstall 16777216  9月 27 18:59 ora_T11_327680_116
-rw-r—–  1 oracle oinstall 16777216  9月 27 18:59 ora_T11_327680_117
-rw-r—–  1 oracle oinstall 16777216  9月 27 18:59 ora_T11_327680_118
-rw-r—–  1 oracle oinstall 16777216  9月 27 18:59 ora_T11_327680_119
-rw-r—–  1 oracle oinstall        0  9月 27 18:59 ora_T11_327680_12

可以看到启动后出现的16MB文件形式共享内存中部分大小为0,这些块被选出当发生内存交换时来被’destory’.使用pmap工具仍可以看到该部分影射,而实际上已经被Oracle释放了.

现在我们加大pga,观察其交换情况.

SQL> alter system set pga_aggregate_target=1900M ;

System altered.

[oracle@rh2 ~]$ ls -l /dev/shm
总用量 289984
-rw-r—–  1 oracle oinstall 16777216  9月 27 18:59 ora_T11_327680_0
-rw-r—–  1 oracle oinstall 16777216  9月 27 18:59 ora_T11_327680_1
-rw-r—–  1 oracle oinstall        0  9月 27 18:59 ora_T11_327680_10
-rw-r—–  1 oracle oinstall        0  9月 27 19:09 ora_T11_327680_100
-rw-r—–  1 oracle oinstall        0  9月 27 19:09 ora_T11_327680_101
-rw-r—–  1 oracle oinstall        0  9月 27 19:09 ora_T11_327680_102
-rw-r—–  1 oracle oinstall        0  9月 27 19:09 ora_T11_327680_103
-rw-r—–  1 oracle oinstall 16777216  9月 27 18:59 ora_T11_327680_104
-rw-r—–  1 oracle oinstall 16777216  9月 27 18:59 ora_T11_327680_105
-rw-r—–  1 oracle oinstall 16777216  9月 27 18:59 ora_T11_327680_106
-rw-r—–  1 oracle oinstall 16777216  9月 27 18:59 ora_T11_327680_107
-rw-r—–  1 oracle oinstall 16777216  9月 27 18:59 ora_T11_327680_108
-rw-r—–  1 oracle oinstall 16777216  9月 27 18:59 ora_T11_327680_109
-rw-r—–  1 oracle oinstall        0  9月 27 18:59 ora_T11_327680_11
-rw-r—–  1 oracle oinstall        0  9月 27 19:09 ora_T11_327680_110
-rw-r—–  1 oracle oinstall        0  9月 27 19:09 ora_T11_327680_111
-rw-r—–  1 oracle oinstall        0  9月 27 19:09 ora_T11_327680_112
-rw-r—–  1 oracle oinstall        0  9月 27 19:09 ora_T11_327680_113
-rw-r—–  1 oracle oinstall        0  9月 27 19:09 ora_T11_327680_114
-rw-r—–  1 oracle oinstall        0  9月 27 19:09 ora_T11_327680_115
-rw-r—–  1 oracle oinstall        0  9月 27 19:09 ora_T11_327680_116
-rw-r—–  1 oracle oinstall        0  9月 27 19:09 ora_T11_327680_117
-rw-r—–  1 oracle oinstall        0  9月 27 19:09 ora_T11_327680_118
-rw-r—–  1 oracle oinstall        0  9月 27 19:09 ora_T11_327680_119
-rw-r—–  1 oracle oinstall        0  9月 27 18:59 ora_T11_327680_12
-rw-r—–  1 oracle oinstall        0  9月 27 19:09 ora_T11_327680_120
-rw-r—–  1 oracle oinstall        0  9月 27 19:09 ora_T11_327680_121

可以看到出现了大量size为0的"文件",期许的交换出现了.

可见在11g中Oracle采用了新的共享内存实现方式,区别于旧的"一块式"共享段,更为灵活了.

如何找出Oracle中需要或值得重建的索引

This script determines whether an index is a good candidate for a rebuild or for
a bitmap index.  All indexes for a given schema or for a subset of schema’s are
analyzed (except indexes under SYS and SYSTEM)

Instructions

Execution Environment:
<SQL, SQL*Plus, iSQL*Plus>

Access Privileges:
Requires DBA privileges in order to be executed.

Usage:
sqlplus <user>/<pw> @rebuild.index.sql

Instructions:
Copy the script into the file ind_an.sql. Execute the script from SQL*Plus connected
with a user with DBA privileges.  The script requires to parameters:

1. Name of the output file where the report while be generated
2. Name of the SCHEMA to be analyzed.

PROOFREAD THIS SCRIPT BEFORE USING IT! Due to differences in the way text
editors, e-mail packages, and operating systems handle text formatting (spaces,
tabs, and carriage returns), this script may not be in an executable state
when you first receive it. Check over the script to ensure that errors of
this type are corrected.

Description

This script determines whether an index is a good candidate for a rebuild or for
a bitmap index.  All indexes for a given schema or for a subset of schema’s are
analyzed (except indexes under SYS and SYSTEM).

REM =============================================================
REM
REM                         rebuild_indx.sql
REM
REM  Copyright (c) Oracle Software, 1998 - 2000
REM
REM  Author  : Jurgen Schelfhout
REM
REM  The sample program in this article is provided for educational
REM  purposes only and is NOT supported by Oracle Support Services.
REM  It has been tested internally, however, and works as documented.
REM  We do not guarantee that it will work for you, so be sure to test
REM  it in your environment before relying on it.
REM
REM  This script will analyze all the indexes for a given schema
REM  or for a subset of schema's. After this the dynamic view
REM  index_stats is consulted to see if an index is a good
REM  candidate for a rebuild or for a bitmap index.
REM
REM  Database Version : 7.3.X and above.
REM
REM  NOTE:  If running this on 10g, you must exclude the
REM  objects in the Recycle Bin
REM        cursor c_indx is
REM          select owner, table_name, index_name
REM            from dba_indexes
REM           where owner like upper('&schema')
REM             and table_name not like 'BIN$%'
REM             and owner not in ('SYS','SYSTEM');
REM
REM  Additional References for Recycle Bin functionality:
REM  Note.265254.1 Flashback Table feature in Oracle Database 10g
REM  Note.265253.1 10g Recyclebin Features And How To Disable it(_recyclebin)
REM
REM =============================================================

prompt
ACCEPT spoolfile CHAR prompt 'Output-file : ';
ACCEPT schema CHAR prompt 'Schema name (% allowed) : ';
prompt
prompt
prompt Rebuild the index when :
prompt   - deleted entries represent 20% or more of the current entries
prompt   - the index depth is more then 4 levels.
prompt Possible candidate for bitmap index :
prompt   - when distinctiveness is more than 99%
prompt

spool &spoolfile;
set serveroutput on;
set verify off;
set linesize 140;
declare
  c_name        INTEGER;
  ignore        INTEGER;
  height        index_stats.height%TYPE := 0;
  lf_rows       index_stats.lf_rows%TYPE := 0;
  del_lf_rows   index_stats.del_lf_rows%TYPE := 0;
  distinct_keys index_stats.distinct_keys%TYPE := 0;
  cursor c_indx is
    select owner, table_name, index_name
      from dba_indexes
     where owner like upper('&schema')
       and owner not in ('SYS', 'SYSTEM');
begin
  dbms_output.enable(1000000);
  dbms_output.put_line('Owner           Index Name                              % Deleted Entries Blevel Distinctiveness');
  dbms_output.put_line('--------------  ---------------------------------            ------------  -----           -----');

  c_name := DBMS_SQL.OPEN_CURSOR;
  for r_indx in c_indx loop
    DBMS_SQL.PARSE(c_name,
                   'analyze index ' || r_indx.owner || '.' ||
                   r_indx.index_name || ' validate structure',
                   DBMS_SQL.NATIVE);
    ignore := DBMS_SQL.EXECUTE(c_name);

    select HEIGHT,
           decode(LF_ROWS, 0, 1, LF_ROWS),
           DEL_LF_ROWS,
           decode(DISTINCT_KEYS, 0, 1, DISTINCT_KEYS)
      into height, lf_rows, del_lf_rows, distinct_keys
      from index_stats;
    /*
    - Index is considered as candidate for rebuild when :
    -   - when deleted entries represent 20% or more of the current entries
    -   - when the index depth is more then 4 levels.(height starts counting from 1 so > 5)
    - Index is (possible) candidate for a bitmap index when :
    -   - distinctiveness is more than 99%
    */
    if (height > 5) OR ((del_lf_rows / lf_rows) > 0.2) then
      dbms_output.put_line(rpad(r_indx.owner, 16, ' ') ||
                           rpad(r_indx.index_name, 40, ' ') ||
                           lpad(round((del_lf_rows / lf_rows) * 100, 3),
                                17,
                                ' ') || lpad(height - 1, 7, ' ') ||
                           lpad(round((lf_rows - distinct_keys) * 100 /
                                      lf_rows,
                                      3),
                                16,
                                ' '));
    end if;

  end loop;
  DBMS_SQL.CLOSE_CURSOR(c_name);
end;
/
spool off;
set verify on;

Sample Output:

SQL> @rebuild_index

Output-file : index_rebuild
Schema name (% allowed) : maclean

Rebuild the index when :
- deleted entries represent 20% or more of the current entries
- the index depth is more then 4 levels.
Possible candidate for bitmap index :
- when distinctiveness is more than 99%

Owner           Index Name                              % Deleted Entries Blevel Distinctiveness
--------------  ---------------------------------            ------------  -----           -----
MACLEAN         SYS_MTABLE_00000CFD4_IND_2                             25      0              25
MACLEAN         SYS_MTABLE_00000D3F3_IND_2                         33.333      0          33.333
PL/SQL procedure successfully completed.

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