当fast_start_parallel_rollback= Low/High

oracle dbahttp://www.cuug.com/ fast_start_parallel_rollback参数决定了SMON在回滚事务时使用的并行度，若将该参数设置为false那么并行回滚将被禁用，若设置为Low(默认值)那么会以2*CPU_COUNT数目的并行度回滚，当设置为High则4*CPU_COUNT数目的回滚进程将参与进来。当我们通过以下查询发现系统中存在大的dead tranacation需要回滚时我们可以通过设置fast_start_parallel_rollback为HIGH来加速恢复:

select sum(distinct(ktuxesiz)) from x$ktuxe where ktuxecfl = 'DEAD';

 

==============parallel transaction recovery===============

 

*** 2011-06-24 20:31:01.765

SMON: system monitor process posted msgflag:0x0000 (-/-/-/-/-/-/-)

 

*** 2011-06-24 20:31:01.765

SMON: process sort segment requests begin

 

*** 2011-06-24 20:31:01.765

SMON: process sort segment requests end

 

*** 2011-06-24 20:31:01.765

SMON: parallel transaction recovery begin

WAIT #0: nam='DFS lock handle' ela= 504 type|mode=1413545989 id1=3 id2=11 obj#=2 tim=1308918661765715

WAIT #0: nam='DFS lock handle' ela= 346 type|mode=1413545989 id1=3 id2=12 obj#=2 tim=1308918661766135

WAIT #0: nam='DFS lock handle' ela= 565 type|mode=1413545989 id1=3 id2=13 obj#=2 tim=1308918661766758

WAIT #0: nam='DFS lock handle' ela= 409 type|mode=1413545989 id1=3 id2=14 obj#=2 tim=1308918661767221

WAIT #0: nam='DFS lock handle' ela= 332 type|mode=1413545989 id1=3 id2=15 obj#=2 tim=1308918661767746

WAIT #0: nam='DFS lock handle' ela= 316 type|mode=1413545989 id1=3 id2=16 obj#=2 tim=1308918661768146

WAIT #0: nam='DFS lock handle' ela= 349 type|mode=1413545989 id1=3 id2=17 obj#=2 tim=1308918661768549

WAIT #0: nam='DFS lock handle' ela= 258 type|mode=1413545989 id1=3 id2=18 obj#=2 tim=1308918661768858

WAIT #0: nam='DFS lock handle' ela= 310 type|mode=1413545989 id1=3 id2=19 obj#=2 tim=1308918661769224

WAIT #0: nam='DFS lock handle' ela= 281 type|mode=1413545989 id1=3 id2=20 obj#=2 tim=1308918661769555

 

*** 2011-06-24 20:31:01.769

SMON: parallel transaction recovery end

 

但是在real world的实践中可以发现当fast_start_parallel_rollback= Low/High，即启用并行回滚时常有并行进程因为各种资源互相阻塞导致回滚工作停滞的例子，当遭遇到这种问题时将fast_start_parallel_rollback设置为FALSE一般可以保证恢复工作以串行形式在较长时间内完成。