有多少Undo enqueue resource? 每一个rollback segment对应一个 US enqueue 资源  

有多少US enqueue lock？ 当进程要求访问一个rollback segment RBS时对应一个US enqueue lock  

谁使用该enqueue lock? 所有的前台进程，SMON和PMON  

何时使用该US enqueue ? US,Undo Segment enqueue的存在是为了对给定undo segment(rollback segment)的串行化DDL，该US enqueue 串行化以下操作： - CREATE ROLLBACK SEGMENT - DROP ROLLBACK SEGMENT - ALTER ROLLBACK SEGMENT ONLINE - ALTER ROLLBACK OFFLINE - ALTER ROLLBACK SEGMENT SHRINK - ALTER ROLLBACK SEGMENT STORAGE - Offlining PENDING OFFLINE RBS by SMON - SMON - abortive offline cleanup. - STARTUP.   注意在UNDO_MANAGEMENT=AUTO的 AMU模式下当Undo Tablespace可用空间紧张时也可能引起剧烈的enq: US队列锁争用等待事件。  

Id1, Id2 组合: Undo Segment#, Always 0.

Lock Value Block: No.

Init.ora Parameters: gc_rollback_locks, rollback_segments.

Scope: Local and Global Enqueue.

Deadlock Sensitive: Yes.

Operations: Synchronous.