FAST_START_PARALLEL_ROLLBACK specifies the degree of parallelism used when recovering terminated transactions. Terminated transactions are transactions that are active before a system failure. If a system fails when there are uncommitted parallel DML or DDL transactions, then you can speed up transaction recovery during startup by using this parameter.

關於fast_start_parallel_rollback引數，此引數決定了回滾啟動的並行次數，在繁忙的系統或者IO效能較差的系統，如果出現大量回滾操作，會顯著影響系統，可以通過調節此引數來降低影響

VALUES：

   false：parallel rollback is disabled

   low：limits the maximum degree of parallelim to 2*cpu_count

   high：limits the maximum degree of parallelim to 4*cpu_count

If you change the value of this parameter, then transaction recovery will be stopped and restarted with the new implied degree of parallelism.

else：另外將fast_start_parallel_rollback時，建議將parallel_force_local調整為true，以限制所有的程序都只執行在當前例項，不至於拖垮其他的節點。

回滾時間計算：

  alter session set NLS_DATE_FORMAT='DD-MON-YYYY HH24:MI:SS'; 
  select usn,
         state,
         undoblockstotal "Total",
         undoblocksdone "Done",
         undoblockstotal - undoblocksdone "ToDo",
         decode(cputime,
                0,
                'unknown',
                sysdate + (((undoblockstotal - undoblocksdone) /
                (decode(UNDOBLOCKSDONE,0,1,UNDOBLOCKSDONE) / cputime)) / 86400)) "Estimated time to complete"
from v$fast_start_transactions
where state!='RECOVERED';
 

注意：對於v$fast_recovery_transactions並不會記錄所有的回滾工作，一般的操作引起的回滾，如rollback,中斷操作，是不會記錄的。這個檢視一般記錄的是大量的操作期間，例項異常關閉或會話被killed之後引起的回滾，裡面記錄了回滾操作的狀態，執行時間，操作的資料塊，程序id等