Redo allocation latch介绍

在Log Buffer中分配内存空间时需要获取Redo allocation latch 。
在Oracle9.2之前，Redo allocation latch 是唯一的，因此向Log Buffer Cache中写入redo entries时是串行的。
在Oracle9.2企业版中，Redo allocation latch 的数量由LOG_PARALLELISM控制。
我们可以查询V$LATCH_CHILDREN视图获得Latch信息:

10:02:41 SQL> select * from v$version;

BANNER
----------------------------------------------------------------
Oracle9i Enterprise Edition Release 9.2.0.6.0 - Production
PL/SQL Release 9.2.0.6.0 - Production
CORE    9.2.0.6.0       Production
TNS for 32-bit Windows: Version 9.2.0.6.0 - Production
NLSRTL Version 9.2.0.6.0 - Production

10:02:46 SQL> select name,latch#,child#,gets,misses
10:02:51   2  from v$latch_children where name = 'redo allocation';

NAME                               LATCH#     CHILD#       GETS     MISSES
------------------------------ ---------- ---------- ---------- ----------
redo allocation                       115          1        815          1

Redo allocation latch 在Log Buffer中为每个事务分配空间，如果事务很小或者服务器只有一个CPU，Redo allocation latch 同时COPY事务数据到Log Buffer Cache。在Log Switch释放空间时，Redo allocation latch 和 Redo Copy Latch同时被释放，也即在Log Switch过程中，Redo的生成是被禁止的。
在Oracle10g中，为了减少竞争，Oracle缺省的预分配19个redo allocation Latch：

SQL> select * from v$version; BANNER ---------------------------------------------------------------- Oracle Database 10g Enterprise Edition Release 10.1.0.3.0 - Prod PL/SQL Release 10.1.0.3.0 - Production CORE 10.1.0.3.0 Production TNS for 32-bit Windows: Version 10.1.0.3.0 - Production NLSRTL Version 10.1.0.3.0 - Production SQL> SQL> select name,latch#,child#,gets,misses 2 from v$latch_children where name = 'redo allocation'; NAME LATCH# CHILD# GETS MISSES ------------------------------ ---------- ---------- ---------- ---------- redo allocation 137 1 1193 1 redo allocation 137 2 111 0 redo allocation 137 3 399 0 redo allocation 137 4 7 0 redo allocation 137 5 4 0 redo allocation 137 6 4 0 redo allocation 137 7 4 0 redo allocation 137 8 4 0 redo allocation 137 9 4 0 redo allocation 137 10 4 0 redo allocation 137 11 4 0 NAME LATCH# CHILD# GETS MISSES ------------------------------ ---------- ---------- ---------- ---------- redo allocation 137 12 4 0 redo allocation 137 13 4 0 redo allocation 137 14 4 0 redo allocation 137 15 4 0 redo allocation 137 16 4 0 redo allocation 137 17 4 0 redo allocation 137 18 4 0 redo allocation 137 19 4 0 19 rows selected.