Oracle 10g Redo的增强：
在Oracle 10g中，log_parallelism参数变为隐含参数，并且Oracle引入了另外两个参数，允许log_parallelism进行动态调整。缺省的_log_parallelism_dynamic参数被设置为True，如果_log_parallelism_max被设置为不同于_log_parallelism的参数值，那么Oracle会动态的选择并行度，当然不超过最大允许值，这是Oracle 10g中动态SGA的另外一个提高。
sys@TQGZS> @GetHidPar.sql
Enter value for par: log_parallelism
old 4: AND x.ksppinm LIKE '%&par%'
new 4: AND x.ksppinm LIKE '%log_parallelism%'
NAME VALUE DESCRIB
------------------------------ ----------- -------------------------------------
_log_parallelism 1 Number of log buffer strands
_log_parallelism_max 1 Maximum number of log buffer strands
_log_parallelism_dynamic TRUE Enable dynamic strands
但是需要注意的是当日志并行度被设置大于1之后，LogMiner将不能解析日志文件，ORA-01374号错误提示说明了这个问题：
[oracle@tqgzs: ~]$oerr ora 01374
01374, 00000, "_log_parallelism_max greater than 1 not supported in this release"
// *Cause: LogMiner does not mine redo records generated with
// _log_parallelism_max set to a value greater than 1.
// *Action:
//
相较Oracle 9i中的Public Redolog Strands（缩写为PBRS），Oracle 10gR2中更引入了Private Redolog Strands的概念（缩写为PVRS），在PVRS机制下，数据库可以在共享池中分配大量的小的私有内存，通常每个大小在64～128KB左右，被独立的Redo Allocation Latch所保护，当数据库中特定类型的小事务开始时会被绑定到独立且空闲PVRS，每个Buffer绑定一个活动事务。在新的机制下，Redo产生后可以直接存入PVRS，而不再保存在PGA中，这样就不再需要额外的内存拷贝过程，Redo Copy Latch也就不再需要（PVRS也因此被称为ZERO-COPY Redo），而Redo Copy正是引发Redo Allocation Latch竞争的根源。
在新的机制下，在进行Redo写出工作时，LGWR需要做的工作就是将PBRS和PVRS中的内容写出，当Redo Flush发生时，所有的Public Redo Allocation Latch需要被获取，所有Public Strands的Redo Copy Latch需要被检查，所有包含活动事务的Private Strands需要被持有。
以下是PVRS在共享池中的内存分配信息（注意在RAC环境中不适用PVRS）：
sys@TQGZS> select banner from x$version where indx in (0,3);
BANNER
----------------------------------------------------------------
Oracle Database 10g Enterprise Edition Release 10.2.0.1.0 - Prod
TNS for Linux: Version 10.2.0.1.0 - Production
sys@TQGZS> select * from v$sgastat where name='private strands';
POOL NAME BYTES
------------ ------------------------------ ----------
shared pool private strands 1198080
在Oracle 10gR2中，Oracle通过使用多个Redo Allocation Latch来提高并发性能：
sys@TQGZS> select addr,latch#,child#,name,gets,immediate_gets,immediate_misses
2 from v$latch_children where name='redo allocation';
ADDR LATCH# CHILD# NAME GETS IMMEDIATE_GETS IMMEDIATE_MISSES
-------- ---------- ---------- -------------------- ---------- -------------- ----------------
2FAD48A8 148 19 redo allocation 5 0 0
2FAD4844 148 18 redo allocation 5 0 0
2FAD47E0 148 17 redo allocation 5 0 0
2FAD477C 148 16 redo allocation 5 0 0
2FAD4718 148 15 redo allocation 5 0 0
2FAD46B4 148 14 redo allocation 5 0 0
2FAD4650 148 13 redo allocation 5 0 0
2FAD45EC 148 12 redo allocation 5 0 0
2FAD4588 148 11 redo allocation 5 0 0
2FAD4524 148 10 redo allocation 5 0 0
2FAD44C0 148 9 redo allocation 5 0 0
2FAD445C 148 8 redo allocation 5 0 0
2FAD43F8 148 7 redo allocation 5 0 0
2FAD4394 148 6 redo allocation 5 0 0
2FAD4330 148 5 redo allocation 5 0 0
2FAD42CC 148 4 redo allocation 615 0 0
2FAD4268 148 3 redo allocation 285 0 0
2FAD4204 148 2 redo allocation 391 0 0
2FAD41A0 148 1 redo allocation 1552 12281 2
19 rows selected.
以下是与PVRS相关的几个隐含参数，缺省的_log_private_parallelism被设置为FALSE：
sys@TQGZS> @GetHidPar.sql
Enter value for par: log_private
old 4: AND x.ksppinm LIKE '%&par%'
new 4: AND x.ksppinm LIKE '%log_private%'
NAME VALUE DESCRIB
---------------------------- --------- --------------------------------------------------------------
_log_private_parallelism FALSE Number of private log buffer strands for zero-copy redo
_log_private_parallelism_mul 10 Active sessions multiplier to deduce number of private strands
_log_private_mul 5 Private strand multiplier for log space preallocation
PVRS的统计数据可以从v$sysstat视图查询得到，由于IMU（In Memory Undo）与PVRS紧密相关，所以两者的信息具有相关性：
sys@TQGZS> select name,value from v$sysstat where upper(name) like '%IMU%';
NAME VALUE
---------------------------------------- -----------
doubling up with imu segment 0
IMU commits 167
IMU Flushes 94
IMU contention 0
IMU recursive-transaction flush 0
IMU undo retention flush 0
IMU ktichg flush 0
IMU bind flushes 0
IMU mbu flush 0
IMU pool not allocated 0
IMU CR rollbacks 6
IMU undo allocation size 2453800
IMU Redo allocation size 758612
IMU- failed to get a private strand 0
OS Maximum resident set size 0
15 rows selected.
在实施了PVRS之后，在警告日志信息中可能看到如下信息，这里的Private_strands就是新特性引入的新提示：
Tue Mar 11 06:07:02 2008
Private_strands 7 at log switch
Thread 1 advanced to log sequence 1228431
Current log# 2 seq# 1228431 mem# 0: /databases/app/oracle/oradata/r2gdb/redo02.log
更进一步地，如果收到提示Private strand flush not complet，这是指从内存到Redo Log File的写出尚未完成：
Mon Nov 30 16:25:06 2009
Thread 1 advanced to log sequence 2589
Current log# 3 seq# 2589 mem# 0: /home/oracle/app/oracle/oradata/ccdb/redo03.log
Thread 1 cannot allocate new log, sequence 2590
Private strand flush not complete
Current log# 3 seq# 2589 mem# 0: /home/oracle/app/oracle/oradata/ccdb/redo03.log
Thread 1 advanced to log sequence 2590
Current log# 1 seq# 2590 mem# 0: /home/oracle/app/oracle/oradata/ccdb/redo01.log
通过v$event_name可以找到关于这个新等待的说明：
sys@TQGZS> select name from v$event_name where upper(name) like '%STRAND%';
NAME
--------------------------------------------------
log file switch (private strand flush incomplete)
- The End -