问题描述
Oracle 11.2.0.4 的DG备库的数据库警告日志显示：
如何理解同样的 seq# 10566 写入到两个不同路径的REDO文件中？
mem# 0 mem# 1 如何理解？
为什么提示Thread 1 cannot allocate new log, sequence 10565，但是新数据却写入到了REDO 中？
sql
Mon Nov 24 05:43:27 2025
Thread 1 cannot allocate new log, sequence 10564
Private strand flush not complete
Current log# 3 seq# 10563 mem# 0: /u01/app/oracle/oradata/wewin/redo03a.log
Current log# 3 seq# 10563 mem# 1: /u01/app/oracle/fast_recovery_area/wewin/redo03b.log
Thread 1 advanced to log sequence 10564 (LGWR switch)
Current log# 4 seq# 10564 mem# 0: /u01/app/oracle/oradata/wewin/redo04a.log
Current log# 4 seq# 10564 mem# 1: /u01/app/oracle/fast_recovery_area/wewin/redo04b.log
Mon Nov 24 05:43:39 2025
Archived Log entry 39090 added for thread 1 sequence 10563 ID 0xdc596ccd dest 1:
Mon Nov 24 05:49:32 2025
Thread 1 cannot allocate new log, sequence 10565
Private strand flush not complete
Current log# 4 seq# 10564 mem# 0: /u01/app/oracle/oradata/wewin/redo04a.log
Current log# 4 seq# 10564 mem# 1: /u01/app/oracle/fast_recovery_area/wewin/redo04b.log
Thread 1 advanced to log sequence 10565 (LGWR switch)
Current log# 5 seq# 10565 mem# 0: /u01/app/oracle/oradata/wewin/redo05a.log
Current log# 5 seq# 10565 mem# 1: /u01/app/oracle/fast_recovery_area/wewin/redo05b.log
Mon Nov 24 05:49:41 2025
Archived Log entry 39091 added for thread 1 sequence 10564 ID 0xdc596ccd dest 1:
Mon Nov 24 08:20:31 2025
Thread 1 advanced to log sequence 10566 (LGWR switch)
Current log# 6 seq# 10566 mem# 0: /u01/app/oracle/oradata/wewin/redo06a.log
Current log# 6 seq# 10566 mem# 1: /u01/app/oracle/fast_recovery_area/wewin/redo06b.log
Mon Nov 24 08:20:38 2025
Archived Log entry 39092 added for thread 1 sequence 10565 ID 0xdc596ccd dest 1:1.REDO日志多路复用机制
1.1 mem# 0和mem# 1的含义
根据《Oracle 11.2 Concepts》文档：
mem# 0 和 mem# 1 表示同一个日志组的不同成员（members）
这是Oracle的多路复用（multiplexing） 机制，用于提高REDO日志的可靠性
同一个日志序列号（seq#）会同时写入到所有成员文件中。
1.2 多路复用的作用
-- 查看当前REDO日志组配置
sql
col group# for a5
col member for a40
col type for a5
col is_recovery_dest_file for a10
SELECT group#, member, type, is_recovery_dest_file
FROM v$logfile
ORDER BY group#, member;
-- 示例输出可能为：
-- GROUP# MEMBER TYPE IS_RECOVERY_DEST_FILE
-- 3 /u01/app/oracle/oradata/wewin/redo03a.log ONLINE NO
-- 3 /u01/app/oracle/fast_recovery_area/wewin/redo03b.log ONLINE YES设计目的：
数据安全：防止单点故障，如果一个成员损坏，另一个成员仍可用
高可用性：确保REDO日志的完整性
符合最佳实践：Oracle强烈推荐多路复用REDO日志
1.3 "Private strand flush not complete"机制分析
Private Strand技术背景
根据《Oracle Core Essential Internals for DBAs and Developers.pdf》文档：
-- Private Strand是Oracle 10g引入的优化机制
-- 每个会话有私有的REDO区域（约64KB/128KB）
-- 减少公共REDO日志缓冲区的争用
2. 日志切换过程详解
正常日志切换流程：
LGWR写满当前日志文件
尝试切换到下一个日志组
如果下一个日志组可用，立即切换
写入新的序列号出现"Private strand flush not complete"时的流程：
LGWR写满当前日志文件（seq# 10564）
尝试切换到序列号10565的日志组
发现私有REDO线程（private strand）尚未完全刷新
记录警告"Thread 1 cannot allocate new log, sequence 10565"
等待private strand刷新完成
刷新完成后，成功切换到seq# 10565
继续正常写入3. 为什么提示无法分配却还能写入？
-- 这是一个"暂时性阻塞"，不是永久性失败
-- Oracle的日志切换机制包含重试逻辑
具体时序分析：
05:49:32 - 第一次尝试切换seq#10565 → 失败（private strand未就绪）
05:49:32 - 等待private strand刷新（约几毫秒到几秒）
05:49:32 - private strand刷新完成
05:49:32 - 第二次尝试切换 → 成功
05:49:32 - 开始写入seq#105654. 问题诊断和解决方案
4.1 诊断Private Strand刷新问题
检查Private Strand状态
-- 检查private strand相关统计信息
col name for a50
SELECT name, value
FROM v$sysstat
WHERE name LIKE '%strand%' OR name LIKE '%private%';
NAME VALUE
-------------------------------------------------- ----------
IMU- failed to get a private strand 0
IM space private journal extents allocated 0
IM space private journal bytes allocated 0
IM space private journal extents freed 0
IM space private journal bytes freed 0
IM space private journal segments allocated 0
IM space private journal segments freed 0
-- 检查等待事件
SELECT event, total_waits, time_waited
FROM v$system_event
WHERE event LIKE '%strand%' OR event LIKE '%private%';
检查日志切换频率和性能（常用脚本，建议收藏）
-- 分析日志切换间隔
select trunc(completion_time) as "Data",
round((sum(blocks*block_size))/1024/1024/1024,0) as "GB"
from v$archived_log
group by trunc(completion_time)
order by 1;
Data GB
------------------- ----------
2024-09-09 00:00:00 17
2024-09-10 00:00:00 4
2024-09-11 00:00:00 0
2024-09-12 00:00:00 4
2024-10-11 00:00:00 6
2024-10-12 00:00:00 2
2024-10-18 00:00:00 2
2024-11-06 00:00:00 0
2024-11-07 00:00:00 0
2024-11-08 00:00:00 0
2025-06-13 00:00:00 0
2025-06-14 00:00:00 0
2025-09-09 00:00:00 0
2025-10-23 00:00:00 0
2025-10-24 00:00:00 0
2025-11-04 00:00:00 0
2025-11-18 00:00:00 0
-- 检查日志文件大小是否合适
SELECT group#, bytes/1024/1024 as size_mb, members, status
FROM v$log;
GROUP# SIZE_MB MEMBERS STATUS
---------- ---------- ---------- --------------------------------
1 50 1 INACTIVE
2 50 1 CURRENT
3 50 1 INACTIVE5. 解决方案
5.1 方案1：调整日志文件大小（推荐）
根据《Oracle 11.2 Performance Tuning Guide.pdf》建议：
-- 如果日志切换过于频繁（小于5分钟），需要增大日志文件
-- 当前日志切换间隔：10564→10565约6分钟，基本合理但偏小
-- 建议将日志文件增大到至少1GB
ALTER DATABASE ADD LOGFILE GROUP 7
('/u01/app/oracle/oradata/wewin/redo07a.log',
'/u01/app/oracle/fast_recovery_area/wewin/redo07b.log')
SIZE 1G;
-- 切换后删除旧的小日志组
ALTER SYSTEM SWITCH LOGFILE;
-- 等待所有日志组状态为INACTIVE后删除
ALTER DATABASE DROP LOGFILE GROUP 1;5.2 方案2：优化Private Strand参数
-- 检查当前private strand设置
SELECT name, value
FROM v$parameter
WHERE name LIKE '%strand%' OR name LIKE '%imu%';
-- 如果频繁出现private strand问题，可以考虑调整
set pages 1000 lines 180
col name for a60
col value for a15
select x.ksppinm name,
y.ksppstvl value,
y.ksppstdf isdefault,
decode(bitand(y.ksppstvf,7),1,'MODIFIED',4,'SYSTEM_MOD','FALSE') ismod,
decode(bitand(y.ksppstvf,2),2,'TRUE','FALSE') isadj
from
sys.x$ksppi x,sys.x$ksppcv y
where
x.inst_id = userenv('Instance')
and y.inst_id = userenv('Instance')
and x.indx = y.indx
and x.ksppinm in ('_log_private_parallelism_mul')
order by translate(x.ksppinm, ' _', ' ');
NAME VALUE ISDEFAULT ISMOD ISADJ
-------------------------------- --------------- ------------------ -------------------- ----------
_log_private_parallelism_mul 10 TRUE FALSE FALSE5.3 方案3：监控和优化I/O性能
-- 检查REDO日志写入性能
SELECT event, time_waited_micro, total_waits,
time_waited_micro/total_waits as avg_wait_micro
FROM v$system_event
WHERE event IN ('log file parallel write', 'log file sync');
EVENT TIME_WAITED_MICRO TOTAL_WAITS AVG_WAIT_MICRO
------------------------- ----------------- ----------- --------------
log file parallel write 195098815 5837 33424.5015
log file sync 1970974 75 26279.6533
-- 如果平均等待时间过高，需要优化存储
-- log file parallel write > 10ms 需要关注
-- log file sync > 5ms 需要优化
预防措施
建立监控告警
当天每小时的各个实例归档日志生成量（常用脚本，建议收藏）
select THREAD#,logtime,count(*),round((sum(blocks*block_size)/1024/1024/1024),2) size_GB
from (select THREAD#,to_char(first_time,'yyyy-mm-dd hh24') as logtime,a.BLOCKS,a.BLOCK_SIZE from v$archived_log a
where a.DEST_ID=1 and a.FIRST_TIME> trunc(sysdate))
group by THREAD#,logtime
order by THREAD#,logtime;
THREAD# LOGTIME COUNT(*) SIZE_GB
---------- -------------------------- ---------- ----------
1 2025-11-18 11 3 .01
定期检查日志配置
-- 每月检查一次日志配置合理性
SELECT
group#,
sequence#,
bytes/1024/1024 as size_mb,
members,
status,
(SELECT COUNT(*) FROM v$log_history WHERE sequence# BETWEEN l.sequence#-10 AND l.sequence#) as recent_switches
FROM v$log l;
GROUP# SEQUENCE# SIZE_MB MEMBERS STATUS RECENT_SWITCHES
---------- ---------- ---------- ---------- -------------- ---------------
1 967 50 1 INACTIVE 11
2 968 50 1 CURRENT 10
3 966 50 1 INACTIVE 11