关于新参数DB_UNRECOVERABLE_SCN_TRACKING
DB_UNRECOVERABLE_SCN_TRACKING这个参数是从11.2.0.2才有的，看官方文档介绍如下：
| Default value | true |
|---|---|
| Modifiable | ALTER SESSION, |
| Range of values | true | false |
| Basic | No |
| Oracle RAC | Multiple instances must have the same value |
DB_UNRECOVERABLE_SCN_TRACKING enables or disables the tracking of unrecoverable (NOLOGGING) direct-path insert and load operations.
When the value is set to true, updates are made to the controlfile that maintains the V$DATAFILE.UNRECOVERABLE_CHANGE# andV$DATAFILE.UNRECOVERABLE_TIME columns. When the value is set to false, updates are not made to the controlfile. Setting this parameter tofalse may improve performance of direct-path NOLOGGING operations.
----------------此参数默认值为True，如果设置为false，带有nologging的更新操作就不能更新到控制文件了。Performance of the NOLOGGING load operation could be limited by the control file write I/O. 如果设置为flase，将会提升直接路径nologging的性能。 多个实例必须有一样的值。可以从v$datafile的下面几个列可以查到：
UNRECOVERABLE_CHANGE#
UNRECOVERABLE_TIME
FIRST_NONLOGGED_SCN
FIRST_NONLOGGED_TIME
这个参数在11.2.0.2上有BUG，官方发布的BUG如下：
Bug 12360160 - ALTER SYSTEM / SESSION of DB_UNRECOVERABLE_SCN_TRACKING has no effect (文档 ID 12360160.8)
Dynamically changing DB_UNRECOVERABLE_SCN_TRACKING with
ALTER SESSION or ALTER SYSTEM has no effect on the value used.
Rediscovery Notes:
The DB_UNRECOVERABLE_SCN_TRACKING init.ora parameter can be
modified dynamically but the change has no effect; the code
uses the value from instance startup, and this startup value
determines whether the unrecoverable scn/timestamp is recorded
in the controlfile (datafile section) for nologging writes.
Workaround
If DB_UNRECOVERABLE_SCN_TRACKING is set to TRUE at startup,
then event 10359 level 1 can be used to dynamically disable
(then later dynamically enable) unrecoverable scn/timestamp
tracking in the controlfile:
alter system set events '10359 trace name context forever, level 1';
alter system set events '10359 trace name context off';
But, if DB_UNRECOVERABLE_SCN_TRACKING is set to FALSE at
startup, there is no workaround to dynamically over-ride this
setting ie the unrecoverable scn/timestamp will not be tracked.
这个BUG已经在11.2.0.3上已经修复了。