一个新生较忽略的错误，引以为戒！

问题症状：系统数据库（节点1主机发生重启，数据库实例暂时未启动），节点2操作系统负载不高，但是业务反应非常缓慢，观察数据库存在较多等待事件：

  COUNT(*) EVENT

---------- --------------------------------------------------

         1 Streams AQ: qmn coordinator idle wait

         1 Streams AQ: waiting for messages in the queue

         1 Streams AQ: qmn slave idle wait

         1 SQL*Net message to client

         1 DIAG idle wait

         1 Streams AQ: waiting for time management or cleanup

            tasks

         2 control file sequential read

         3 SQL*Net message from dblink

        10 cursor: pin S wait on X

        35 log file switch (archiving needed)

        46 buffer busy waits

       135 row cache lock

       376 enq: SQ - contention

13 rows selected.

问题处理过程：

观察以上查询到的等待事件进行，发生等待最多的enq: SQ - contention和row cache lock看起来和序列有关，结合ASH视图查询，发生等待最多的SQL语句：

SQL> select sql_id,count(*)  from v$active_session_history where event  = 'enq: SQ - contention' and sample_time >= to_date('20150928 15:00:00', 'yyyymmdd hh24:mi:ss') group by sql_id;

SQL_ID                                    COUNT(*)

--------------------------------------- ----------

74whb22kwku28                               172933

fzc102gjarjdx                                40257

SQL> select sql_text from v$sqlarea where sql_id='74whb22kwku28';

SQL_TEXT

--------------------------------------------------------------------------------

select GETOPERID from dual  --->GETOPERID函数也是访问SEQ_MD_DEV_STORAGE.nextval

SQL> select sql_text from v$sqlarea where sql_id='fzc102gjarjdx';

SQL_TEXT

--------------------------------------------------------------------------------

select to_char(SEQ_MD_DEV_STORAGE.nextval) as ID from dual

同时查看了相关序列的信息：

SQL> SET LINE 200

SQL> col SEQUENCE_OWNER for a15

SQL> col SEQUENCE_NAME for a45

SQL> select * from dba_sequences a where sequence_name='SEQ_MD_DEV_STORAGE' and sequence_owner='SXYKCC';

SEQUENCE_OWNER  SEQUENCE_NAME                                  MIN_VALUE  MAX_VALUE INCREMENT_BY CYCL ORDE CACHE_SIZE LAST_NUMBER

--------------- --------------------------------------------- ---------- ---------- ------------ ---- ---- ---------- -----------

SXYKCC          SEQ_MD_DEV_STORAGE                                     1 1.0000E+16            1 N    N            20      781075

序列的Cache看起来比较小，尝试加Cache会被hung住，尝试kill部分会话，kill之后并没有新的会话产生。此时注意到，等待事件情况变化为：

  COUNT(*) EVENT

---------- ---------------------------------------------

        32 log file switch (archiving needed)

        19 buffer busy waits

        14 row cache lock

        12 cursor: pin S wait on X

         2 SQL*Net message from dblink

         2 control file sequential read

         1 SQL*Net message to client

7 rows selected.

等待事件log file switch (archiving needed)通常为日志空间满，alert告警日志信息，并无明显的不能归档信息：

on Sep 28 15:20:03 CST 2015

>>> WAITED TOO LONG FOR A ROW CACHE ENQUEUE LOCK! pid=169

System State dumped to trace file /oracle/admin/pmcpdp/bdump/pmcpdp2_j003_25428018.trc

Mon Sep 28 15:21:33 CST 2015

Error 3135 trapped in 2PC on transaction 18.3.2426591. Cleaning up.

Error stack returned to user:

ORA-03135: connection lost contact

Mon Sep 28 15:22:23 CST 2015

>>> WAITED TOO LONG FOR A ROW CACHE ENQUEUE LOCK! pid=233

Mon Sep 28 15:24:28 CST 2015

>>> WAITED TOO LONG FOR A ROW CACHE ENQUEUE LOCK! pid=288

System State dumped to trace file /oracle/admin/pmcpdp/udump/pmcpdp2_ora_19792100.trc

Mon Sep 28 15:27:01 CST 2015

>>> WAITED TOO LONG FOR A ROW CACHE ENQUEUE LOCK! pid=657

Mon Sep 28 15:29:28 CST 2015

>>> WAITED TOO LONG FOR A ROW CACHE ENQUEUE LOCK! pid=735

System State dumped to trace file /oracle/admin/pmcpdp/udump/pmcpdp2_ora_34930770.trc

Mon Sep 28 15:31:40 CST 2015

>>> WAITED TOO LONG FOR A ROW CACHE ENQUEUE LOCK! pid=742

进一步检查归档路径，空间使用情况：

SQL> archive log list

Database log mode              Archive Mode

Automatic archival             Enabled

Archive destination            /backup

Oldest online log sequence     2987

Next log sequence to archive   2987

Current log sequence           2994

$ df -k

Filesystem    1024-blocks      Free %Used    Iused %Iused Mounted on

/dev/hd4          5242880   2967360   44%    11564     2% /

/dev/hd2         10485760   6689804   37%    61859     4% /usr

/dev/hd9var       2097152   1238092   41%     9023     4% /var

/dev/hd3          5242880   4064048   23%     2957     1% /tmp

/dev/fwdump       1572864   1503456    5%       16     1% /var/adm/ras/platform

/dev/hd1        110100480 109831152    1%      385     1% /home

/dev/hd11admin      524288    523864    1%        5     1% /admin

/proc                   -         -    -         -     -  /proc

/dev/hd10opt      5242880   4872412    8%    10861     1% /opt

/dev/livedump      524288    513072    3%       49     1% /var/adm/ras/livedump

/dev/lv00          524288    507776    4%       18     1% /var/adm/csd

/dev/oralv       41943040  23617468   44%    85100     2% /oracle

/dev/fslv00     104857600 104584732    1%       11     1% /arch02

NFS server cmspdb1 not responding still trying

--此处hung住

检查/backup挂载点相关信息：

/backup:

        dev             = /backup

        vfs             = nfs

        nodename        = cmspdb1

        mount           = true

        options         = cio,rw,bg,hard,nointr,rsize=32768,wsize=32768,proto=tcp,noac,vers=3,timeo=600

可以看到节点2的归档路径，实际是从节点1通过NFS挂载过来的归档路径

问题结论：

因为节点1上的NFS服务，为手动方式启动，重启主机以后，NFS服务未正常启动，所以节点2的归档路径变得不可用，导致归档无法正常进行，阻塞了大量会话。手动启动节点1上的NFS服务后，归档恢复正常，数据库等待事件消失。