调整oracle回滚的速度
回滚的速度快慢通过参数fast_start_parallel_rollback来实现，此参数可以动态调整
关于fast_start_parallel_rollback参数，此参数决定了回滚启动的并行次数，在繁忙的系统或者IO性能较差的系统，如果出现大量回滚操作，会显著影响系统系统，可以通过调整此参数来降低影响。官方文档的定义如下

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.

Values:

    FALSE

    Parallel rollback is disabled

    LOW

    Limits the maximum degree of parallelism to 2 * CPU_COUNT

    HIGH

    Limits the maximum degree of parallelism 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.

回滚过程中，回滚的进度可以通过视图V$FAST_START_TRANSACTIONS来确定

SQL> select usn, state, undoblocksdone, undoblockstotal, CPUTIME, pid,xid, rcvservers from v$fast_start_transactions;

       USN STATE            UNDOBLOCKSDONE UNDOBLOCKSTOTAL    CPUTIME        PID XID              RCVSERVERS
---------- ---------------- -------------- --------------- ---------- ---------- ---------------- ----------
       454 RECOVERED                110143          110143        210            01C600210027E0D9          1
       468 RECOVERED                   430             430         17            01D40000001F3A36        128
       
USN：事务对应的undo段
STATE：事务的状态，可选的值为（BE RECOVERED, RECOVERED, or RECOVERING）       
UNDOBLOCKSDONE：已经完成的undo块
UNDOBLOCKSTOTAL：总的undo数据块
CPUTIME：已经回滚的时间，单位是秒
RCVSERVERS：回滚的并行进程数

补充，查询回滚时间更好的脚本
sys@MS4ADB3(dtydb5)> select  undoblockstotal "Total", undoblocksdone "Done", undoblockstotal-undoblocksdone
  2    "ToDo",decode(cputime,0,'unknown',to_char(sysdate+(((undoblockstotal-undoblocksdone)
  3       / (undoblocksdone / cputime)) / 86400),'yyyy-mm-dd hh24:mi:ss'))
  4      "Estimated time to complete"，to_char(sysdate,'yyyy-mm-dd hh24:mi:ss')
  5      from v$fast_start_transactions;

 Total  MB       Done       ToDo Estimated time to complete             TO_CHAR(SYSDATE,'YYYY-MM-DDHH24:MI:SS'
---------- ---------- ---------- -------------------------------------- --------------------------------------
    36,767      36767          0 2014-03-19 16:59:19                    2014-03-19 16:59:19
     7,209       7209          0 2014-03-19 16:59:19                    2014-03-19 16:59:19
     3,428       3428          0 2014-03-19 16:59:19                    2014-03-19 16:59:19
    34,346       1604      32742 2014-03-19 17:25:31                    2014-03-19 16:59:19