数据库的数据都存放在数据文件当中，当数据库开启时，Oracle怎么完成从数据文件读取数据到内存当中这个初始化过程呢?
SQL> startup mount;ORACLE instance started.Total System Global Area 6680915968 bytesFixed Size 2213936 bytesVariable Size 3758098384 bytesDatabase Buffers 2885681152 bytesRedo Buffers 34922496 bytesDatabase mounted.SQL> alter session set sql_trace=true;SQL> alter database open;
通过上面的开启SQL跟踪参数后，启动数据的信息被记录在trace文件中，之前获取trace文件都是通过gettrcname.sql获取的，11G增加了一个新的视图可以直接获取到trace文件：
select value from v$diag_info where name = 'Default Trace File';VALUE--------------------------------------------------------------------/DBBK/oracle/diag/rdbms/orcl/orcl/trace/orcl_ora_22987.trc
如果直接去查看该文件，估计有点费劲，通过tkprof命令转换成人类可读的模式：
tkprof orcl_ora_22987.trc 1.trc
查看1.trc这个文件可以发现从开启SQL_TRACE开始记录的所有语句内容：
SQL ID: 4tk6t8tfsfqbfPlan Hash: 0alter session set sql_trace=truecall count cpu elapsed disk query current rows------- ------ -------- ---------- ---------- ---------- ---------- ----------Parse 0 0.00 0.00 0 0 0 0Execute 1 0.00 0.00 0 0 0 0Fetch 0 0.00 0.00 0 0 0 0------- ------ -------- ---------- ---------- ---------- ---------- ----------total 1 0.00 0.00 0 0 0 0Misses in library cache during parse: 0Misses in library cache during execute: 1Optimizer mode: ALL_ROWSParsing user id: SYS......SQL ID: a01hp0psv0rrhPlan Hash: 0alter database opencall count cpu elapsed disk query current rows------- ------ -------- ---------- ---------- ---------- ---------- ----------Parse 1 0.00 0.00 0 0 0 0Execute 1 0.32 1.45 19 34 63 0Fetch 0 0.00 0.00 0 0 0 0------- ------ -------- ---------- ---------- ---------- ---------- ----------total 2 0.32 1.45 19 34 63 0Misses in library cache during parse: 1Optimizer mode: ALL_ROWSParsing user id: SYS
从发出open语句后开始查看：
SQL ID: 32r4f1brckzq1Plan Hash: 0create table bootstrap$ ( line# number not null, obj# number not null, sql_text varchar2(4000) not null) storage (initial 50K objno 59 extents (file 1 block 520))call count cpu elapsed disk query current rows------- ------ -------- ---------- ---------- ---------- ---------- ----------Parse 1 0.00 0.00 0 0 0 0Execute 1 0.00 0.00 0 0 0 0Fetch 0 0.00 0.00 0 0 0 0------- ------ -------- ---------- ---------- ---------- ---------- ----------total 2 0.00 0.00 0 0 0 0Misses in library cache during parse: 1Optimizer mode: CHOOSEParsing user id: SYS (recursive depth: 1)
这一步，Oracle在内存中创建了一个bootstrap$的表结构，并从file 1 block 520(数据文件1的520号块)内读取数据到该表中。
到这里我们可以查看一下file 1 block 520中记录的对象
SQL> select segment_name,file_id,block_id from dba_extents where file_id=1 and block_id=520;SEGMENT_NAME FILE_ID BLOCK_ID-------------------- ---------- ----------BOOTSTRAP$ 1 520
我们再来看看这张表中的字段和内容：
select line#,obj#,sql_text from bootstrap$; LINE# OBJ# SQL_TEXT---------- ---------- -------------------------------------------------- -1 -1 8.0.0.0.0 0 0 CREATE ROLLBACK SEGMENT SYSTEM STORAGE ( INITIAL 112K NEXT 56K MINEXTENTS 1 MAXEXTENTS 32765 OBJNO 0 EXTENTS (FILE 1 BLOCK 128)) 2 2 CREATE CLUSTER C_OBJ#("OBJ#" NUMBER) PCTFREE 5 PCT USED 40 INITRANS 2 MAXTRANS 255 STORAGE ( INITIAL 136K NEXT 200K MINEXTENTS 1 MAXEXTENTS 2147483645 PCTINCREASE 0 OBJNO 2 EXTENTS (FILE 1 BLOCK 144)) SIZE 800 3 3 CREATE INDEX I_OBJ# ON CLUSTER C_OBJ# PCTFREE 10 I NITRANS 2 MAXTRANS 255 STORAGE ( INITIAL 64K NEXT 1024K MINEXTENTS 1 MAXEXTENTS 2147483645 PCTINCRE ASE 0 OBJNO 3 EXTENTS (FILE 1 BLOCK 168)) 4 4 CREATE TABLE TAB$("OBJ#" NUMBER NOT NULL,"DATAOBJ# " NUMBER,"TS#" NUMBER NOT NULL,"FILE#" NUMBER NOT NULL,"BLOCK#" NUMBER NOT NULL,"BOBJ#" NUMBER,"TAB# " NUMBER,"COLS" NUMBER NOT NULL,"CLUCOLS" NUMBER," PCTFREE$" NUMBER NOT NULL,"PCTUSED$" NUMBER NOT NU LL,"INITRANS" NUMBER NOT NULL,"MAXTRANS" NUMBER NO T NULL,"FLAGS" NUMBER NOT NULL,"AUDIT$" VARCHAR2(3 8) NOT NULL,"ROWCNT" NUMBER,"BLKCNT" NUMBER,"EMPCN T" NUMBER,"AVGSPC" NUMBER,"CHNCNT" NUMBER,"AVGRLN" NUMBER,"AVGSPC_FLB" NUMBER,"FLBCNT" NUMBER,"ANALY ZETIME" DATE,"SAMPLESIZE" NUMBER,"DEGREE" NUMBER," INSTANCES" NUMBER,"INTCOLS" NUMBER NOT NULL,"KERNE LCOLS" NUMBER NOT NULL,"PROPERTY" NUMBER NOT NULL, "TRIGFLAG" NUMBER,"SPARE1" NUMBER,"SPARE2" NUMBER, "SPARE3" NUMBER,"SPARE4" VARCHAR2(1000),"SPARE5" V ARCHAR2(1000),"SPARE6" DATE) STORAGE ( OBJNO 4 TA BNO 1) CLUSTER C_OBJ#(OBJ#) 5 5 CREATE TABLE CLU$("OBJ#" NUMBER NOT NULL,"DATAOBJ# " NUMBER,"TS#" NUMBER NOT NULL,"FILE#" NUMBER NOT NULL,"BLOCK#" NUMBER NOT NULL,"COLS" NUMBER NOT NU LL,"PCTFREE$" NUMBER NOT NULL,"PCTUSED$" NUMBER NO T NULL,"INITRANS" NUMBER NOT NULL,"MAXTRANS" NUMBE R NOT NULL,"SIZE$" NUMBER,"HASHFUNC" VARCHAR2(30), "HASHKEYS" NUMBER,"FUNC" NUMBER,"EXTIND" NUMBER,"F LAGS" NUMBER,"DEGREE" NUMBER,"INSTANCES" NUMBER,"A VGCHN" NUMBER,"SPARE1" NUMBER,"SPARE2" NUMBER,"SPA RE3" NUMBER,"SPARE4" NUMBER,"SPARE5" VARCHAR2(1000 ),"SPARE6" VARCHAR2(1000),"SPARE7" DATE) STORAGE ( OBJNO 5 TABNO 2) CLUSTER C_OBJ#(OBJ#)...... 可以看到该表中又包含了其它创建在内容中的对象,到这里，我们回过头来看SQL_TRACE文件：
SQL ID: 6apq2rjyxmxpjPlan Hash: 867914364select line#, sql_textfrom bootstrap$ where obj# != :1call count cpu elapsed disk query current rows------- ------ -------- ---------- ---------- ---------- ---------- ----------Parse 1 0.00 0.00 0 0 0 0Execute 1 0.00 0.00 0 0 0 0Fetch 60 0.00 0.00 4 61 0 59------- ------ -------- ---------- ---------- ---------- ---------- ----------total 62 0.00 0.00 4 61 0 59Misses in library cache during parse: 1Misses in library cache during execute: 1Optimizer mode: CHOOSEParsing user id: SYS (recursive depth: 1)Rows Row Source Operation------- --------------------------------------------------- 59 TABLE ACCESS FULL BOOTSTRAP$ (cr=61 pr=4 pw=0 time=0 us)********************************************************************************SQL ID: 864bmh11c121aPlan Hash: 0CREATE ROLLBACK SEGMENT SYSTEM STORAGE ( INITIAL 112K NEXT 56K MINEXTENTS 1 MAXEXTENTS 32765 OBJNO 0 EXTENTS (FILE 1 BLOCK 128))call count cpu elapsed disk query current rows------- ------ -------- ---------- ---------- ---------- ---------- ----------Parse 1 0.00 0.00 0 0 0 0Execute 1 0.00 0.00 0 0 0 0Fetch 0 0.00 0.00 0 0 0 0------- ------ -------- ---------- ---------- ---------- ---------- ----------total 2 0.00 0.00 0 0 0 0Misses in library cache during parse: 1Optimizer mode: CHOOSEParsing user id: SYS (recursive depth: 1)......
到这里我想，大家大概就能看懂了，跟我刚才查看bootstrap$一样，数据库去查找该表中的内容，逐一运行 (:1是系统给定的变量，估计该变量值为1)。
即：Oracle启动先将Bootstrap$加载到内存中，随后逐一将数据初始化所需的对象加载到内存中，在BootStrap$表中的对象，每一个都值得研究，这里暂时不详细说明。
通过上述说明，可以看出bootstrap$的重要性，当然在书中还做了一个小测试，就是改动该表中的内容，看看对数据库启动是否造成影响(轻易不要做这样的测试，做之前最好备份数据库，当然这个测试也是可以恢复的)：
SQL> update bootstrap$ set sql_text='9.0.0.1.0' where obj#=-1;1 row updated.SQL> select line#,obj#,sql_text from bootstrap$ where obj#=-1 order by obj#; LINE# OBJ# SQL_TEXT---------- ---------- -------------------------------------------------- -1 -1 9.0.0.1.0SQL> commit;Commit complete.SQL> shutdown immediate;SQL> startup;ORACLE instance started.Total System Global Area 6680915968 bytesFixed Size 2213936 bytesVariable Size 3758098384 bytesDatabase Buffers 2885681152 bytesRedo Buffers 34922496 bytesDatabase mounted.ORA-01092: ORACLE instance terminated. Disconnection forcedORA-00704: bootstrap process failureORA-00702: bootstrap verison '9.0.0.1.0' inconsistent with version '8.0.0.0.0'Process ID: 4883Session ID: 96 Serial number: 3
可以看到数据库mount没问题，但open的时候读取bootstrap$报错，错误内容页写的很清楚，处理bootstrap$失败，版本号不一致。所以对于这类问题的解决方式，最好是时长对数据库进行备份，当问题发生时进行不完全恢复。
通过Oracle自带工具BBED(BLOCK BROWSER/EDITOR)工具进行恢复(关于BBED的工具的编译安装，我的博客中有记载：，这里不详细介绍)
增加配置文件：
指定块大小，修改模式和BBED连接数据文件列表[oracle@021Y-SH-BKAP oracle]$ cat par.bbd blocksize=8192listfile=file.lstmode=edit指定文件列表中的数据文件格式： 数据文件号 路径 大小[oracle@021Y-SH-BKAP oracle]$ cat file.lst 1 /DBBK/oracle/oradata/orcl/system01.dbf 723525632用bbed工具访上面的参数文件(BBED默认密码blockedit)[oracle@021Y-SH-BKAP oracle]$ bbed parfile=par.bbd Password: *********
进行如下操作，因为我们已知了bootstrap是存放在file 1 block 520上的(我这里有点犯傻，就一直在该块和前面的块找改动的内容，实际记录的内容应该向后移动一个块)：
BBED: Release 2.0.0.0.0 - Limited Production on Thu Jan 15 15:56:23 2015Copyright (c) 1982, 2009, Oracle and/or its affiliates. All rights reserved.************* !!! For Oracle Internal Use only !!! ***************BBED> set dba 1,521 offset 0 DBA 0x00400209 (4194825 1,521) OFFSET 0
定位被自己修改的内容(关键要找到内容所在的块)
BBED> find /c 9.0.0.1.0 File: /DBBK/oracle/oradata/orcl/system01.dbf (1) Block: 521 Offsets: 8181 to 8191 Dba:0x00400209---------------------------------------------------------------------- 302e302e 312e3002 0676c9
这里用的是ASCII码，显示的内容明显为0.0.1.0 结果不正确，应该向前偏移2个单位
BBED> dump /v dba 1,521 offset 8179 count 11 File: /DBBK/oracle/oradata/orcl/system01.dbf (1) Block: 521 Offsets: 8179 to 8189 Dba:0x00400209------------------------------------------------------- 392e302e 302e312e 300206 l 9.0.0.1.0..
修改该内容，恢复为8.0.0.0.0
BBED> modify /c '8.0.0.0.0' dba 1,521 offset 8179 File: /DBBK/oracle/oradata/orcl/system01.dbf (1) Block: 521 Offsets: 8179 to 8189 Dba:0x00400209------------------------------------------------------------------------ 382e302e 302e302e 300206BBED> dump /v dba 1,521 offset 8179 count 11 File: /DBBK/oracle/oradata/orcl/system01.dbf (1) Block: 521 Offsets: 8179 to 8189 Dba:0x00400209------------------------------------------------------- 382e302e 302e302e 300206 l 8.0.0.0.0.. <16 bytes per line>BBED> sum dba 1,521 applyCheck value for File 1, Block 521:current = 0x5f51, required = 0x5f51
应用后可以看到current的值与required的值相同。再次启动正常：
SQL> startup ORACLE instance started.Total System Global Area 6680915968 bytesFixed Size 2213936 bytesVariable Size 3758098384 bytesDatabase Buffers 2885681152 bytesRedo Buffers 34922496 bytesDatabase mounted.Database opened.
参考：
《DBA入门，诊断与进阶案例》 盖国强著