Oracle OS Block Header

oracle文件的第一个块（block 0）是OS block header，在数据库中查询不到信息，记录的是OS信息，以及文件大小的等信息：

SQL> select file_name,bytes from dba_data_files;   FILE_NAME                                               BYTES -------------------------------------------------- ---------- /u01/app/oracle/oradata/PROD/user01.dbf              67108864   $ls -lrt total 1390268 -rw-r-----  1 oracle oinstall  67117056 Apr 12 09:31 user01.dbf

从上面可以看出，OS上的大小比数据库里的大小多了一个BLOCK。

如果OS block header损坏，并不影响数据库打开、使用，但重建控制文件时会报错，用dbverify/rman也检测不到坏块，不过可以使用dbfsize来查看：
正常状态：

$dbfsize user01.dbf   Database file: user01.dbf Database file type: file system Database file size: 8192 8192 byte blocks

损坏：

$dbfsize user01.dbf user01.dbf: Header block magic number is bad

编缉BLOCK 0，模拟损坏，可以正常启动、使用：

SQL> startup; ORACLE instance started.   Total System Global Area  184549376 bytes Fixed Size                  1266488 bytes Variable Size             100666568 bytes Database Buffers           79691776 bytes Redo Buffers                2924544 bytes Database mounted. ORA-01113: file 4 needs media recovery ORA-01110: data file 4: '/u01/app/oracle/oradata/PROD/user01.dbf'     SQL> recover datafile 4;  Media recovery complete. SQL> alter database open;   Database altered.   SQL> create table test01 tablespace USERS as select * from dba_objects;   Table created.

用dbv检查，未发现坏块：

$dbv file=user01.dbf   DBVERIFY: Release 10.2.0.4.0 - Production on Mon Apr 16 16:38:33 2012   Copyright (c) 1982, 2007, Oracle.  All rights reserved.   DBVERIFY - Verification starting : FILE = user01.dbf     DBVERIFY - Verification complete   Total Pages Examined         : 8192 Total Pages Processed (Data) : 357 Total Pages Failing   (Data) : 0 Total Pages Processed (Index): 0 Total Pages Failing   (Index): 0 Total Pages Processed (Other): 11 Total Pages Processed (Seg)  : 0 Total Pages Failing   (Seg)  : 0 Total Pages Empty            : 7824 Total Pages Marked Corrupt   : 0 Total Pages Influx           : 0 Highest block SCN            : 336969 (0.336969)

用dbfsize检查，报错：

$dbfsize user01.dbf user01.dbf: Header block magic number is bad

如果重建控制文件，则会报错：

SQL> startup nomount; ORACLE instance started.   Total System Global Area  184549376 bytes Fixed Size                  1266488 bytes Variable Size             100666568 bytes Database Buffers           79691776 bytes Redo Buffers                2924544 bytes SQL> CREATE CONTROLFILE REUSE DATABASE "PROD" NORESETLOGS  ARCHIVELOG   2      MAXLOGFILES 5   3      MAXLOGMEMBERS 5   4      MAXDATAFILES 100   5      MAXINSTANCES 2   6      MAXLOGHISTORY 292   7  LOGFILE   8    GROUP 1 (   9      '/u01/app/oracle/oradata/PROD/REDO1_1.log', 10      '/u01/app/oracle/oradata/PROD/REDO1_2.log', 11      '/u01/app/oracle/oradata/PROD/REDO1_3.log' 12    ) SIZE 100M, 13    GROUP 2 ( 14      '/u01/app/oracle/oradata/PROD/REDO2_1.log', 15      '/u01/app/oracle/oradata/PROD/REDO2_2.log', 16      '/u01/app/oracle/oradata/PROD/REDO2_3.log' 17    ) SIZE 100M 18  -- STANDBY LOGFILE 19  DATAFILE 20    '/u01/app/oracle/oradata/PROD/SYSTEM01.dbf', 21    '/u01/app/oracle/oradata/PROD/undotbs01.dbf', 22    '/u01/app/oracle/oradata/PROD/SYSAUX01.dbf', 23    '/u01/app/oracle/oradata/PROD/user01.dbf' 24  CHARACTER SET AL32UTF8 25  ; CREATE CONTROLFILE REUSE DATABASE "PROD" NORESETLOGS  ARCHIVELOG * ERROR at line 1: ORA-01503: CREATE CONTROLFILE failed ORA-01565: error in identifying file '/u01/app/oracle/oradata/PROD/user01.dbf' ORA-27047: unable to read the header block of file Additional information: 2

报ORA-27047错误。这里可以在数据库打开状态下，resize datafile，这样就可以重写OS block header信息：

SQL> alter database open;   Database altered.   SQL> alter database datafile '/u01/app/oracle/oradata/PROD/user01.dbf' resize 65M;   Database altered.   SQL> select file_name,bytes from dba_data_files;   FILE_NAME                                               BYTES -------------------------------------------------- ---------- /u01/app/oracle/oradata/PROD/user01.dbf              68157440

dbfsize检查正常，重建控制文件正常：

$dbfsize user01.dbf   Database file: user01.dbf Database file type: file system Database file size: 8320 8192 byte blocks   SQL> CREATE CONTROLFILE REUSE DATABASE "PROD" NORESETLOGS  ARCHIVELOG   2      MAXLOGFILES 5   3      MAXLOGMEMBERS 5   4      MAXDATAFILES 100   5      MAXINSTANCES 2   6      MAXLOGHISTORY 292   7  LOGFILE   8    GROUP 1 (   9      '/u01/app/oracle/oradata/PROD/REDO1_1.log', 10      '/u01/app/oracle/oradata/PROD/REDO1_2.log', 11      '/u01/app/oracle/oradata/PROD/REDO1_3.log' 12    ) SIZE 100M, 13    GROUP 2 ( 14      '/u01/app/oracle/oradata/PROD/REDO2_1.log', 15      '/u01/app/oracle/oradata/PROD/REDO2_2.log', 16      '/u01/app/oracle/oradata/PROD/REDO2_3.log' 17    ) SIZE 100M 18  -- STANDBY LOGFILE 19  DATAFILE 20    '/u01/app/oracle/oradata/PROD/SYSTEM01.dbf', 21    '/u01/app/oracle/oradata/PROD/undotbs01.dbf', 22    '/u01/app/oracle/oradata/PROD/SYSAUX01.dbf', 23    '/u01/app/oracle/oradata/PROD/user01.dbf' 24  CHARACTER SET AL32UTF8 25  ;   Control file created.   SQL> alter database open;   Database altered.

注：resize 原大小，重建还是报错，需要resize一个不同的大小。