--测试环境11g,获lgwr的spid
SQL> select * from v$process where username='SYSTEM' and program like '%LGWR%';
 
ADDR                    PID SPID                     PNAME USERNAME           SERIAL# TERMINAL         PROGRAM                                                          TRACEID                                                                          TRACEFILE                                                                        BACKGROUND LATCHWAIT        LATCHSPIN        PGA_USED_MEM PGA_ALLOC_MEM PGA_FREEABLE_MEM PGA_MAX_MEM
---------------- ---------- ------------------------ ----- --------------- ---------- ---------------- ---------------------------------------------------------------- -------------------------------------------------------------------------------- -------------------------------------------------------------------------------- ---------- ---------------- ---------------- ------------ ------------- ---------------- -----------
000007FF614CD768         11 17428                    LGWR  SYSTEM                   1 123-PC           ORACLE.EXE (LGWR)                                                                                                                                 d:\oracle11g_64bit\diag\rdbms\second\second\trace\second_lgwr_17428.trc          1                                                 5007762       5465314           131072     9921762

--另启一会话窗口,暂挂lgwr进程
SQL> oradebug setsospid 17428
ORA-00070: command setsospid is not valid
SQL> oradebug setospid 17428
Oracle pid: 11, Windows thread id: 17428, image: ORACLE.EXE (LGWR)
SQL> oradebug suspend
Statement processed.

--发生日志切换hang住
SQL> alter system switch logfile;
 
System altered
--新建一个会话窗口
create table t_h(a int);--语句hang住

--解挂lgwr进程,上述lgwr及create操作顺利执行完毕
SQL> oradebug resume
Statement processed.

小结：如果日志切换时,会暂住一切所有的数据库前端事务create and dml