数据库突然无法登陆，只有sysdba可以登陆。

检查alert文件：

Fri Jan? 8 11:41:40 2010
Thread 2 advanced to log sequence 2263 (LGWR switch)
Current log# 4 seq# 2263 mem# 0: /oradata/gamis/redo04.log
Fri Jan? 8 12:00:11 2010
Trace dumping is performing id=[cdmp_20100108120010]
Fri Jan? 8 12:30:21 2010
Thread 2 advanced to log sequence 2264 (LGWR switch)
Current log# 5 seq# 2264 mem# 0: /oradata/gamis/redo05.log
Fri Jan? 8 13:10:29 2010
Database in quiesce mode? <============进入
Fri Jan? 8 13:10:30 2010
ALTER SYSTEM SET service_names=’GAMIS’ SCOPE=MEMORY SID=’GAMIS2′;

<==================变化
Fri Jan? 8 13:51:14 2010
Thread 2 advanced to log sequence 2265 (LGWR switch)
Current log# 6 seq# 2265 mem# 0: /oradata/gamis/redo06.log
Fri Jan? 8 14:12:04 2010
Database out of quiesce mode
Fri Jan? 8 14:12:07 2010
ALTER SYSTEM SET service_names=’GAMIS’,'GAMIS_TAF’ SCOPE=MEMORY SID=’GAMIS2′;
Fri Jan? 8 14:13:07 2010
Errors in file /u01/oracle/admin/GAMIS/udump/gamis2_ora_905460.trc:
ORA-00604: error occurred at recursive SQL level 1
ORA-12805: parallel query server died unexpectedly
ORA-06512: at line 10

可以看到在13：10分进入了quiesce mode，所以导致系统没有办法登陆。从随后的更新可以看出，这种更新基本应该由管理工具引起。通过确认，在此刻有工程师在进行emca操作，可以认为是emca引起该问题。

确认文档：

In this Document
Symptoms
Cause
Solution

Applies to:

Enterprise Manager Grid Control – Version: 10.2.0.1 to 11.1.0.7
This problem can occur on any platform.

Symptoms

When running ‘emca -config dbcontrol db -repos recreate’ or ‘emca -deconfig dbcontrol db -repos drop’?to recreate or drop?the Enterprise Manager Database Control configuration files and repository the operation hangs and at the same time users are not able to login to the database.

Cause

The emca command, when recreating or dropping the repository, requires the database to be quiesced.??This?means that, temporarily, no operations are possible on the database and?new users cannot login.

If there are active sessions on the database at the time the quiesce operation will hang pending their completion, during which time no new users will be able to connect.

Therefore it is important to understand that running these commands may/will cause the database to be unavailable during the time that the repository is being dropped and should not be run on a production database unless you are fully aware of the possible impact to database availability and have planned for this eventuality

Solution

To implement the solution, please execute the following steps:

1. Kill the emca command
2. Connect to the database with SYS account through SQL*Plus
SQL > alter system unquiesce;

Run the emca command at a time when there is low system usage.
For additional information on emca, please consult:
Enterprise Manager Advanced Configuration
Topic 1.2.6 Configuring Database Control During and After the Oracle Database 10g Installation
Available on OTN at:
http://download.oracle.com/docs/cd/B16240_01/doc/nav/portal_booklist.htm