Oracle DG（Dataguard）是目前比较常见的数据库HA配置策略。通过实现Physical Standby和Logical Standby，可以实现数据冗余容错机制。防止在主库出现严重故障，不能支持服务的时候，没有快速的后备支持环境。
在DG中，switchover和failover是两个重要的概念，也是DG实现的核心。两者共同点都是Primary和Standby角色切换，差异在于Planned和UnPlanned之分。Switchover关键点在于Planned，这个切换动作是在运维机构规划范围内的动作。比如，进行定期系统软硬件升级、设备维修等动作。而Failover是真正出现严重系统故障，如数据库宕机、软硬件故障导致的Primary不能支持服务，从而进行的切换动作。
根据不同的DG配置，switchover和failover也是有差异的。理论上，Switchover是不会造成数据丢失的，Primary在切换之后也是在DG配置环境中，作为Standby存在的。但是Failover则不同，除了运行在最大保护（Maximum Protection）模式下，Primary突发的故障可能引起一部分Redo Log不能及时的传递到Standby端，切换之后很可能有数据损失的情况。更重要的是，Primary端在发生Failover之后，是不能够直接加入回DG配置的！也就是说，Failover之后，Primary实际上就是被“抛出”了DG环境。
那么，有什么方法实现Primary回到原有的环境呢？这个问题的困难在于保持Primary和Standby一致。在正常情况下，Primary和Standby之间是关联同步的，即使发生了Switchover，也在可控情况下。Failover过程中有数据的缺失，还有Primary修复问题。在目前流行版本（11g）中，有三个方法：
ü 环境重建：一种最简单的方法就是直接删除原来的Primary库，引用DG重建方法，重新搭建Standby端；
ü RMAN备份恢复：如果Primary端保留过一份Failover之前的备份，则可以强制原来的Primary端恢复到进行Failover的时间点，之后作为Standby接收当前Primary的redo log传递，应用后可以跟上进度；
ü Flashback Database恢复：Flashback技术是作为传统备份还原技术的补充，提供了更加便捷的恢复策略。使用flashback，可以将数据库恢复到failover之前的时间点。之后的过程和RMAN备份恢复策略相同；
案例分析：
一、在主库端模拟数据库意外宕机
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scott@bjdb>conn /
as
sysdba
Connected.
sys@bjdb>alter system switch logfile;
System altered.
sys@bjdb>shutdown abort
ORACLE instance shut down.
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二、在备库端
1、查看切换信息
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sys@shdb>select name,database_role,switchover_status
from
v$database;
NAME DATABASE_ROLE SWITCHOVER_STATUS
--------- ---------------- --------------------
TESTDB12 PHYSICAL STANDBY NOT ALLOWED
可以看到此时备库处于无法切换状态
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2、直接切换
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sys@shdb>alter database commit to switchover to primary;
alert_log:（告警日志）
Fatal NI connect error
12514
, connecting to:
(DESCRIPTION=(ADDRESS_LIST=(ADDRESS=(PROTOCOL=TCP)(HOST=shsrv)(PORT=
1521
)))(CONNECT_DATA=(SERVICE_NAME=shdb)(CID=(PROGRAM=oracle)(HOST=bjsrv)(USER=oracle))))
VERSION INFORMATION:
TNS
for
Linux: Version
11.2.
0.3.
0
- Production
TCP/IP NT Protocol Adapter
for
Linux: Version
11.2.
0.3.
0
- Production
Time:
04
-MAR
-2015
21
:
25
:
13
Tracing
not
turned
on
.
Tns error struct:
ns main err code:
12564
TNS
-12564
: TNS:connection refused
ns secondary err code:
0
nt main err code:
0
nt secondary err code:
0
nt OS err code:
0
Error
12514
received logging
on
to the standby
FAL[client, MRP0]: Error
12514
connecting to shdb
for
fetching gap sequence
Wed Mar
04
21
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26
:
00
2015
alter database commit to switchover to primary
ALTER DATABASE SWITCHOVER TO PRIMARY (TestDB12)
Maximum wait
for
role transition is
15
minutes.
Switchover: Media recovery is still active
Database
not
available
for
switchover
End-Of-REDO archived log file has
not
been recovered
Database
not
available
for
switchover
End-Of-REDO archived log file has
not
been recovered
Database
not
available
for
switchover
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3、关闭standby MPR进程
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sys@shdb>ALTER DATABASE RECOVER managed standby database finish;
ALTER DATABASE RECOVER managed standby database finish
Terminal Recovery: request posted (TestDB12)
Wed Mar
04
21
:
34
:
34
2015
Begin: Standby Redo Logfile archival
End: Standby Redo Logfile archival
Terminal Recovery timestamp is
'03/04/2015 21:34:34'
Terminal Recovery: applying standby redo logs.
Terminal Recovery: thread
1
seq#
34
redo required
Media Recovery Waiting
for
thread
1
sequence
34
Terminal Recovery: End-Of-Redo log allocation
Terminal Recovery: standby redo logfile
4
created
'/dsk4/arch_bj/arch_1_0_820054583.log'
This standby redo logfile is being created
as
part of the
failover operation. This standby redo logfile should be
deleted
after
the switchover to primary operation completes.
Media Recovery Log /dsk4/arch_bj/arch_1_0_820054583.log
Terminal Recovery: log
4
reserved
for
thread
1
sequence
34
Recovery of Online Redo Log: Thread
1
Group
4
Seq
34
Reading mem
0
Mem#
0
: /dsk4/arch_bj/arch_1_0_820054583.log
Identified End-Of-Redo (failover)
for
thread
1
sequence
34
at
SCN
0xffff.
ffffffff
Incomplete Recovery applied until change
1234252
time
03
/
04
/
2015
21
:
23
:
43
MRP0: Media Recovery Complete (TestDB12)
Terminal Recovery: successful completion
Wed Mar
04
21
:
34
:
35
2015
ARCH: Archival stopped, error occurred. Will
continue
retrying
ORACLE Instance TestDB12 - Archival Error
ORA
-16014
: log
4
sequence#
34
not
archived, no available destinations
ORA
-00312
: online log
4
thread
1
:
'/dsk4/arch_bj/arch_1_0_820054583.log'
Forcing ARSCN to IRSCN
for
TR
0
:
1234252
Attempt to set limbo arscn
0
:
1234252
irscn
0
:
1234252
Resetting standby activation ID
2865247982
(
0xaac836ee
)
MRP0: Background Media Recovery process shutdown (TestDB12)
Terminal Recovery: completion detected (TestDB12)
Completed: ALTER DATABASE RECOVER managed standby database finish
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4、切换数据库到Primary
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sys@shdb>select status
from
v$instance;
STATUS
------------
OPEN
sys@shdb>select name,database_role,switchover_status
from
v$database;
NAME DATABASE_ROLE SWITCHOVER_STATUS
--------- ---------------- --------------------
TESTDB12 PHYSICAL STANDBY TO PRIMARY
sys@shdb>alter database commit to switchover to primary;
Database altered.
sys@shdb>alter database open;
Database altered.
告警日志：
alter database commit to switchover to primary
ALTER DATABASE SWITCHOVER TO PRIMARY (TestDB12)
Maximum wait
for
role transition is
15
minutes.
All dispatchers
and
shared servers shutdown
CLOSE: killing server sessions.
CLOSE: all sessions shutdown successfully.
Wed Mar
04
21
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35
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2015
SMON: disabling cache recovery
Backup controlfile written to trace file /u01/app/oracle/diag/rdbms/bjdb/TestDB12/trace/TestDB12_ora_3146.trc
Standby terminal recovery start SCN:
1234251
RESETLOGS
after
incomplete recovery UNTIL CHANGE
1234252
Online log /dsk2/oradata/bjdb/redo01b.log: Thread
1
Group
1
was previously cleared
Online log /dsk1/oradata/bjdb/redo01a.log: Thread
1
Group
1
was previously cleared
Online log /dsk2/oradata/bjdb/redo02b.log: Thread
1
Group
2
was previously cleared
Online log /dsk1/oradata/bjdb/redo02a.log: Thread
1
Group
2
was previously cleared
Online log /dsk2/oradata/bjdb/redo03b.log: Thread
1
Group
3
was previously cleared
Online log /dsk1/oradata/bjdb/redo03a.log: Thread
1
Group
3
was previously cleared
Standby became primary SCN:
1234250
Wed Mar
04
21
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35
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2015
Setting recovery target incarnation to
3
AUDIT_TRAIL initialization parameter is changed back to its original value
as
specified
in
the parameter file.
Switchover: Complete - Database mounted
as
primary
Completed: alter database commit to switchover to primary
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三、原主库修复后，开机
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sys@bjdb>startup
ORACLE instance started.
Total System Global Area
442601472
bytes
Fixed Size
2229184
bytes
Variable Size
281021504
bytes
Database Buffers
155189248
bytes
Redo Buffers
4161536
bytes
Database mounted.
Database opened.
sys@bjdb>select name,database_role,switchover_status
from
v$database;
NAME DATABASE_ROLE SWITCHOVER_STATUS
--------- ---------------- --------------------
TESTDB12 PRIMARY FAILED DESTINATION
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现在原来的主库被修复后，整个DataGuara架构已经被破坏了，所以必须把原来的主库构建成新的备库，重新恢复DataGuard的环境。
四、重新构建DataGuard
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sys@bjdb>select name,database_role
from
v$database;
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NAME DATABASE_ROLE
-------------------------------------------------- ----------------
TESTDB12 PHYSICAL STANDBY
本文转自 客居天涯 51CTO博客，原文链接：http://blog.51cto.com/tiany/1617646，如需转载请自行联系原作者
