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$fast_start_transactions;
total mb done todo estimated time to complete to_char(sysdate,'yyyy-mm-ddhh24:mi:ss'
---------- ---------- ---------- -------------------------------------- --------------------------------------
36,767 36767 0 2014-03-19 16:59:19 2014-03-19 16:59:19
7,209 7209 0 2014-03-19 16:59:19 2014-03-19 16:59:19
3,428 3428 0 2014-03-19 16:59:19 2014-03-19 16:59:19
34,346 1604 32742 2014-03-19 17:25:31 2014-03-19 16:59:19
下面是一次大量wait for a undo record等待事件的处理过程
1，某用户使用plsql执行某 insert操作异常，导致表空间不断增长，于是手工kill该回滚停掉，kill后大量wait for a undo record，大约100多个
2，查询v$fast_start_transactions视图，由于fast_start_parallel_rollback参数设置为high，且cpu为32个，因此并行进程为32×3=128个
sql> select usn, state, undoblocksdone, undoblockstotal, cputime, pid,xid, rcvservers from v$fast_start_transactions;
usn state undoblocksdone undoblockstotal cputime pid xid rcvservers
---------- ---------------- -------------- --------------- ---------- ---------- ---------------- ----------
454 recovering 26922 464160 103 3744 01c600210027e0d9 128
468 recovered 430 430 17 01d40000001f3a36 128
sql> show parameter rollback