概述

最近因为PLSQL崩溃导致几千万的数据产生了回滚，大概有300多万块需要回滚，那么怎么去调整Oracle回滚的速度呢？

fast_start_parallel_rollback

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.

回滚进度

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

select usn, state, undoblocksdone, undoblockstotal, CPUTIME, pid, xid, rcvservers from v$fast_start_transactions;--或者以下sqlselect ktuxeusn USN, ktuxeslt Slot, ktuxesqn Seq, ktuxesta State, ktuxesiz Undo from sys.ktuxe where ktuxesta <> 'INACTIVE' and ktuxecfl like '%DEAD%' order by ktuxesiz asc;

说明：

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：回滚的并行进程数计算回滚时间

通过以下sql可以计算回滚的实际时间，不过不是很准确的，这个时间一般会慢慢变少。

select undoblockstotal "Total", undoblocksdone "Done", undoblockstotal-undoblocksdone"ToDo",decode(cputime,0,'unknown',to_char(sysdate+(((undoblockstotal-undoblocksdone)/ (undoblocksdone / cputime)) / 86400),'yyyy-mm-dd hh24:mi:ss'))"Estimated time to complete"，to_char(sysdate,'yyyy-mm-dd hh24:mi:ss')from v$fast_start_transactions;

为了加快undo回滚的速度，可以通过设置fast_start_parallel_rollback，在线修改，立即生效

alter system set fast_start_parallel_rollback=HIGH scope=both;

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