Oracle 10g新特性之memory undo（imu）

 
  我们知道在Oracle 10g之前，Oracle执行事务之前必须在undo表空间记录事务的执行情况，如分配事务表（TX Table），分配undo block。进行这些操作必须将undo block读至buffer cache，这又会涉及到Oracle latch分配和latch pin等操作。Oracle为了实现一致性读取要求，假如undo block没在buffer cache，又需从undo表空间物理读取block，这些资源密集型操作势必影响其性能。从Oracle 10g开始，Oracle开始推出一新特性memory undo，即专门在shared pool中开辟内存处理事务，以下就简单研究一下In memory UNDO，简称imu，注意在RAC情况下imu不被支持,搜索metalink目前imu存在较多bug

研究平台为

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SQL> select * from v$version;

BANNER
----------------------------------------------------------------
Oracle Database 10g Enterprise Edition Release 10.2.0.3.0 - Prod
PL/SQL Release 10.2.0.3.0 - Production
CORE    10.2.0.3.0      Production
TNS for Linux: Version 10.2.0.3.0 - Production
NLSRTL Version 10.2.0.3.0 - Production

Oralce 10g是否使用由隐含参数_in_memory_undo决定，默认情况为TRUE。

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_in_memory_undo                          TRUE                 Make in memory undo for top level transactions

此外imu还涉及以下参数
imu pool在shard_pool分配的个数，注意此参数和实际分配的个数不符，只作为阀值存在：

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_imu_pools                               3                    in memory undo pools

DBWR是否刷新imu pool：

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_db_writer_flush_imu                     TRUE                 If FALSE, DBWR will not downgrade IMU txns for AGING

递归操作是否启动imu：

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_recursive_imu_transactions              FALSE                recursive transactions may be IMU

基于以上参数我们再深入探索。
查看imu pool在shared_pool分配的大小，可以看到在shared_pool中分配了1235304 BYTES。

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SQL> select * from v$sgastat where name='KTI-UNDO';

POOL         NAME                                          BYTES
------------ ---------------------------------------- ----------
shared pool  KTI-UNDO                                    1235304

进一步通过X$KTIFP，可以查看每一个imu pool内存使用情况，可以看到目前系统中imu pool共为18个（为参数transactions*10%），每个pool大小为65535 bytes（对于32为系统为64k + 24bytes=65535 bytes，对于64位系统为128k + 1k=132096 bytes），可以看到pool大小为64k，所以imu只针对小事务使用。

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SQL> show parameter transactions

NAME                                 TYPE        VALUE
------------------------------------ ----------- ------------------------------
transactions                         integer     187
SQL> select KTIFPNO,KTIFPPSI from X$KTIFP;

   KTIFPNO   KTIFPPSI
---------- ----------
         0      65535
         1      65535
         2      65535
         3         16
         4      65535
         5      65535
         6      65535
         7      65535
         8      65535
         9      65535
        10      65535

   KTIFPNO   KTIFPPSI
---------- ----------
        11      65535
        12      65535
        13      65535
        14      65535
        15      65535
        16      65535
        17      65535

18 rows selected.

Oracle alert日志也可以看到分配给imu pool数量，用ILAT =18表示

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Autotune of undo retention is turned on.
IMODE=BR
ILAT =18
LICENSE_MAX_USERS = 0
SYS auditing is disabled

通过以下视图可以查看Oracle触发imu刷新内存的条件

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SQL> col INDX for 999
SQL> col KTIFFCAT for a50
SQL> select indx,ktiffcat,ktiffflc from X$KTIFF;
INDX KTIFFCAT                                             KTIFFFLC
---- -------------------------------------------------- ----------
   0 Undo pool overflow flushes                                  0
   1 Stack cv flushes                                        34142
   2 Multi-block undo flushes                                    0
   3 Max. chgs flushes                                       51607
   4 NTP flushes                                               492
   5 Contention flushes                                        679
   6 Redo pool overflow flushes                                  1
   7 Logfile space flushes                                       0
   8 Multiple persistent buffer flushes                          0
   9 Bind time flushes                                           0
  10 Rollback flushes                                         4945

INDX KTIFFCAT                                             KTIFFFLC
---- -------------------------------------------------- ----------
  11 Commit flushes                                         215244
  12 Recursive txn flushes                                   49992
  13 Redo only CR flushes                                        0
  14 Ditributed txn flushes                                      0
  15 Set txn use rbs flushes                                     0
  16 Bitmap state change flushes                             85557
  17 Presumed commit violation                                   0

18 rows selected.

当系统启用imu，部分事务即可通过imu提交。

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SQL> select name,value from v$sysstat where name like '%commits%';

NAME                                                                  VALUE
---------------------------------------------------------------- ----------
user commits                                                          42785
IMU commits                                                           40411

当然了Oracle使用imu特性也是通过latch保护的，每一个imu pool都有相应的latch保护。每个事务独立的使用imu pool。

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SQL> select LATCH#,NAME,GETS,MISSES from v$latch where name ='In memory undo latch';

    LATCH# NAME                                           GETS     MISSES
---------- ---------------------------------------- ---------- ----------
       192 In memory undo latch                       14548635         74

SQL> select NAME,GETS,MISSES from v$latch_children where name='In memory undo latch';

NAME                                           GETS     MISSES
---------------------------------------- ---------- ----------
In memory undo latch                        2997946         31
In memory undo latch                         286606          4
In memory undo latch                        6641899         13
In memory undo latch                        4328804         13
In memory undo latch                         113778         13
In memory undo latch                         121081          0
In memory undo latch                          58811          0
In memory undo latch                              0          0
In memory undo latch                              0          0
In memory undo latch                              0          0
In memory undo latch                              0          0

NAME                                           GETS     MISSES
---------------------------------------- ---------- ----------
In memory undo latch                              0          0
In memory undo latch                              0          0
In memory undo latch                              0          0
In memory undo latch                              0          0
In memory undo latch                              0          0
In memory undo latch                              0          0
In memory undo latch                              0          0

18 rows selected.

简单总结：
Oracle 10g通过在shared_pool中开辟一小块内存，用来缓存未提交的小事务相关的undo block，然后通过一定的触发条件，有序批量的写出，有效的避免了不必要的读写操作，对于性能的提高启到了一定的作用。