29-Oracle 23ai Flashback Log Placement（闪回日志灵活配置）

小伙伴们有没有被各种存储路径满导致的业务崩，半夜起来清理的经历。一不小心 FRA写满了，导致了实例hang住。

OCM考试，时不时就会冒出来这个直接给instance hang，本就卡的环境中脑袋都卡壳、无从下手，一脸懵直接崩。

23ai flashback Log Placement解耦存储依赖，重构性能瓶颈，释放灵活性

一、Flashback发展演进

​1. 之前的闪回日志的固定保存路径

2. Oracle 23ai的架构革新：Flashback Log Placement​

-- ASM磁盘组和文件路径
ALTER SYSTEM SET DB_FLASHBACK_LOG_DEST='+DATA_FRA' scope=SPFILE;

ALTER SYSTEM SET DB_FLASHBACK_LOG_DEST='/opt/oracle/FB_FRA' scope=SPFILE;

ALTER SYSTEM SET DB_FLASHBACK_LOG_DEST_SIZE=100G;

二、新特性技术解析：参数与操作

​1. 核心参数说明 可以单独设置独立的存储

1、可以设置到更快的磁盘中，比如SSD

2、降低FRA的压力

3、提高FLASHBACK单独存储空间和FRA的性能。

4、三个参数可以动态设置，RAC环境中所有的节点都要一致，且在CDB$ROOT中设置

3. 查询视图

-- 闪回日志存储状态
-- 路径、空间使用率
SYS@CDB$ROOT> SELECT * FROM v$flashback_log_dest;

NAME       SPACE_LIMIT    SPACE_USED    NUMBER_OF_FILES    CON_ID
_______ ______________ _____________ __________________ _________
                     0                                0         0

-- 闪回窗口有效性  -- 可恢复的最早SCN
SELECT oldest_flashback_scn, retention_target FROM v$flashback_database_log;

-- 性能指标 
SELECT * FROM v$flashback_database_stat;
SYS@CDB$ROOT> SELECT * FROM v$flashback_database_stat;

BEGIN_TIME    END_TIME        FLASHBACK_DATA    DB_DATA    REDO_DATA    ESTIMATED_FLASHBACK_SIZE    CON_ID
_____________ ____________ _________________ __________ ____________ ___________________________ _________
13-JUN-25     13-JUN-25              4038656    2383872      3918848                           0         0

三、23ai 闪回日志独立存储配置脚本​​ 

-- 1. 关闭闪回日志（需MOUNT状态）
SHUTDOWN IMMEDIATE;
STARTUP MOUNT;
ALTER DATABASE FLASHBACK OFF;  -- 关闭闪回

-- 2. 配置独立存储路径与空间
ALTER SYSTEM SET DB_FLASHBACK_LOG_DEST_SIZE=100G SCOPE=SPFILE;
ALTER SYSTEM SET DB_FLASHBACK_LOG_DEST='+DATA_FRA' SCOPE=SPFILE;

-- 3. 重新启用闪回
ALTER DATABASE FLASHBACK ON;  -- 开启闪回
ALTER DATABASE OPEN;

-----
--检查参数
SYS@CDB$ROOT> show parameter flashback;
NAME                          TYPE        VALUE
----------------------------- ----------- -----
db_flashback_log_dest         string
db_flashback_log_dest_size    big integer 0
db_flashback_retention_target integer     1440
-- 步骤1：启用归档模式（必需前提）
SHUTDOWN IMMEDIATE;
STARTUP MOUNT;
ALTER DATABASE ARCHIVELOG;          -- 必须为归档模式
ALTER DATABASE OPEN;
--实操记录
SYS@CDB$ROOT> SHUTDOWN IMMEDIATE;
Database closed.
Database dismounted.
ORACLE instance shut down.
SYS@CDB$ROOT> STARTUP MOUNT;
ORACLE instance started.
Total System Global Area   1603287928 bytes
Fixed Size                    4922232 bytes
Variable Size               872415232 bytes
Database Buffers            721420288 bytes
Redo Buffers                  4530176 bytes
Database mounted.
SYS@CDB$ROOT> ALTER DATABASE ARCHIVELOG;
ALTER DATABASE ARCHIVELOG
SYS@CDB$ROOT> alter database open;

Database altered.

SYS@CDB$ROOT> SELECT LOG_MODE FROM V$DATABASE;

LOG_MODE
_____________
ARCHIVELOG

SYS@CDB$ROOT>
--
SYS@CDB$ROOT> alter system set db_recovery_file_dest_size=50G scope=SPFILE;
System altered.
SYS@CDB$ROOT> alter system set db_recovery_file_dest='/opt/oracle/DB_FRA' scope=SPFILE;
System altered.
-- 步骤2：关闭闪回并配置独立存储（必须按顺序执行）
SHUTDOWN IMMEDIATE;
STARTUP MOUNT;
ALTER DATABASE FLASHBACK OFF;       -- 先关闭闪回
--SYS@CDB$ROOT> STARTUP MOUNT;
ORACLE instance started.

Total System Global Area   1603287928 bytes
Fixed Size                    4922232 bytes
Variable Size               620756992 bytes
Database Buffers            973078528 bytes
Redo Buffers                  4530176 bytes
Database mounted.
SYS@CDB$ROOT> ALTER DATABASE FLASHBACK OFF;
Database altered.
--
-- 必须先设置_SIZE
ALTER SYSTEM SET DB_FLASHBACK_LOG_DEST_SIZE=100G SCOPE=SPFILE;
-- 支持ASM/文件系统
ALTER SYSTEM SET DB_FLASHBACK_LOG_DEST='+DATA_FRA' SCOPE=SPFILE;
ALTER SYSTEM SET DB_FLASHBACK_LOG_DEST='/opt/oracle/FB_FRA' SCOPE=SPFILE;
-- 重新启用闪回
ALTER DATABASE FLASHBACK ON;
SYS@CDB$ROOT> alter database flashback on;
Database altered.

ALTER DATABASE OPEN;
SYS@CDB$ROOT> ALTER DATABASE OPEN;

Database altered.
-- 步骤3：验证配置
-- **23ai新增视图
SYS@CDB$ROOT> desc v$flashback_database_log;

Name                        Null?    Type
___________________________ ________ _________
OLDEST_FLASHBACK_SCN                 NUMBER
OLDEST_FLASHBACK_TIME                DATE
RETENTION_TARGET                     NUMBER
FLASHBACK_SIZE                       NUMBER
ESTIMATED_FLASHBACK_SIZE             NUMBER
CON_ID                               NUMBER
SYS@CDB$ROOT>

SELECT * FROM v$flashback_database_log;
SYS@CDB$ROOT> SELECT * FROM v$flashback_database_log;

   OLDEST_FLASHBACK_SCN OLDEST_FLASHBACK_TIME       RETENTION_TARGET    FLASHBACK_SIZE    ESTIMATED_FLASHBACK_SIZE    CON_ID
_______________________ ________________________ ___________________ _________________ ___________________________ _________
                3294212 13-JUN-25                               1440         419430400                           0         0
-- 确认闪回状态和归档模式
SELECT flashback_on, log_mode FROM v$database;
SYS@CDB$ROOT> SELECT flashback_on, log_mode FROM v$database;

FLASHBACK_ON    LOG_MODE
_______________ _____________
YES             ARCHIVELOG
SYS@CDB$ROOT> show parameter db_flashback;
NAME                          TYPE        VALUE
----------------------------- ----------- ------------------
db_flashback_log_dest         string      /opt/oracle/FB_FRA
db_flashback_log_dest_size    big integer 100G
db_flashback_retention_target integer     1440
SYS@CDB$ROOT>

四、关键约束点

设置flashback DEST和开启flashback之前必须开启归档（这个大家都知道了），

1、参数设置顺序强制约束​，

ORA-02097: parameter cannot be modified because specified value is invalid

ORA-38706: Cannot turn on FLASHBACK DATABASE logging.
ORA-38709: Recovery Area is not enabled.

 2、RAC环境特殊要求​

-- RAC中所有实例配置必须一致
ALTER SYSTEM SET DB_FLASHBACK_LOG_DEST_SIZE=100G SCOPE=SPFILE SID='*';
ALTER SYSTEM SET DB_FLASHBACK_LOG_DEST='+DATA_FRA' SCOPE=SPFILE SID='*';

五、与从前设置的差异

技术本质​：从 ​​“FRA附属日志”​​ 到 ​​“一级恢复基础设施”​​ 的定位跃迁