關於新參數DB_UNRECOVERABLE_SCN_TRACKING，unrecoverablefault

關於新參數DB_UNRECOVERABLE_SCN_TRACKING，unrecoverablefault

DB_UNRECOVERABLE_SCN_TRACKING這個參數是從11.2.0.2才有的，看官方文檔介紹如下：

Default value	true
Modifiable	ALTER SESSION, ALTER SYSTEM
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.

----------------此參數預設值為True，如果設定為false，帶有nologging的更新操作就不能更新到控制檔案了。Performance of the  NOLOGGING load operation could be limited by the control file write I/O. 如果設定為flase，將會提升直接路徑nologging的效能。

多個執行個體必須有一樣的值。可以從v$datafile的下面幾個列可以查到：

UNRECOVERABLE_CHANGE#     
UNRECOVERABLE_TIME     
FIRST_NONLOGGED_SCN 
FIRST_NONLOGGED_TIME

這個參數在11.2.0.2上有BUG，官方發布的BUG如下：

Bug 12360160 - ALTER SYSTEM / SESSION of DB_UNRECOVERABLE_SCN_TRACKING has no effect (文檔 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.

這個BUG已經在11.2.0.3上已經修複了。