文章转自MOS:
Cannot Connect As "/ as sysdba" ORA-27140 ORA-27300 ORA-27301 ORA-27302 And ORA-27303 In Trace File (文档 ID 453959.1)

APPLIES TO:

Oracle Server - Enterprise Edition - Version: 10.2.0.3 to 11.2.0.3 - Release: 10.2 to 11.2
Information in this document applies to any platform.
***Checked for relevance on 12-Jan-2012***

SYMPTOMS

Connect " /as sysdba" is showing connected but failing when performing any query.

	$ sqlplus "/as sysdba"

SQL*Plus: Release 10.2.0.3.0 - Production on Wed Aug 8 19:34:19 2007

Copyright (c) 1982, 2006, Oracle. All Rights Reserved.

Connected.

SQL> select * from v$instance;

select * from v$instance

*

ERROR at line 1:

ORA-01012: not logged on

CHANGES

gid of the $ORACLE_HOME/bin/oracle was changed when the database was running.

CAUSE

At the time of startup the background processes are having a gid, and since the gid of the
$ORACLE_HOME/bin/oracle was changed, the new gid was not matching with the gid at startup.

Below entries can be found in the trace file, ORA-27303 indicates the startup and current gid.

	Oracle Database 10g Enterprise Edition Release 10.2.0.3.0 - 64bit Production 

With the Partitioning, OLAP and Data Mining options 

ORACLE_HOME = /opt/oracle/product/10.2.0.3 

System name: AIX 

Node name: oracleprd1 

Release: 3 

Version: 5 

Machine: 00CA784D4C00 

Instance name: ORCL

Redo thread mounted by this instance: 1 

Oracle process number: 0 

Unix process pid: 524430, image: oracle@oracleprd1 

Died during process startup with error 27140 (seq=805) 

OPIRIP: Uncaught error 27140. Error stack: 

ORA-27140: attach to post/wait facility failed 

ORA-27300: OS system dependent operation:invalid_egid failed with status: 1 

ORA-27301: OS failure message: Not owner 

ORA-27302: failure occurred at: skgpwinit6 

ORA-27303: additional information: startup egid = 202 (?), current egid = 101 (dba) 

oracle@ectrdbp0: BELKPRD : /opt/oracle/BELKPRD/bdump

SOLUTION

Change the gid of the $ORACLE_HOME/bin/oracle to the original value i.e. startup gid.