關於新參數DB_UNRECOVERABLE_SCN_TRACKING

關於新參數DB_UNRECOVERABLE_SCN_TRACKING

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.

----------------此參數默認值為True，如果設置為false，帶有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. 如果設置為flase，將會提升直接路徑nologging的性能。

多個實例必須有一樣的值。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上已經修復了。