数据库HANG住，最后被迫重启。是一个RAC环境，10.2.0.4

1.山西电信告警alert_mega.log

Mon Jun 01 02:50:03 CST 2015
>>> WAITED TOO LONG FOR A ROW CACHE ENQUEUE LOCK! pid=31
System State dumped to trace file /opt/oracle/admin/dba/udump/mega_ora_27190.trc
Tue Jun 02 02:50:03 CST 2015
>>> WAITED TOO LONG FOR A ROW CACHE ENQUEUE LOCK! pid=32
System State dumped to trace file /opt/oracle/admin/dba/udump/mega_ora_2063.trc
Tue Jun 02 17:53:40 CST 2015
>>> WAITED TOO LONG FOR A ROW CACHE ENQUEUE LOCK! pid=37
System State dumped to trace file /opt/oracle/admin/dba/udump/mega_ora_7006.trc
Tue Jun 02 18:00:41 CST 2015
>>> WAITED TOO LONG FOR A ROW CACHE ENQUEUE LOCK! pid=38
System State dumped to trace file /opt/oracle/admin/dba/udump/mega_ora_7120.trc
Wed Jun 03 02:50:02 CST 2015
>>> WAITED TOO LONG FOR A ROW CACHE ENQUEUE LOCK! pid=39
System State dumped to trace file /opt/oracle/admin/dba/udump/mega_ora_9632.trc
Wed Jun 03 20:54:39 CST 2015
>>> WAITED TOO LONG FOR A ROW CACHE ENQUEUE LOCK! pid=41
System State dumped to trace file /opt/oracle/admin/dba/udump/mega_ora_15023.trc
Wed Jun 03 20:55:07 CST 2015
>>> WAITED TOO LONG FOR A ROW CACHE ENQUEUE LOCK! pid=42
System State dumped to trace file /opt/oracle/admin/dba/udump/mega_ora_15029.trc
Wed Jun 03 20:55:26 CST 2015
>>> WAITED TOO LONG FOR A ROW CACHE ENQUEUE LOCK! pid=43
System State dumped to trace file /opt/oracle/admin/dba/udump/mega_ora_15033.trc
Thu Jun 04 02:50:02 CST 2015
>>> WAITED TOO LONG FOR A ROW CACHE ENQUEUE LOCK! pid=44
System State dumped to trace file /opt/oracle/admin/dba/udump/mega_ora_16725.trc
Fri Jun 05 02:50:02 CST 2015
>>> WAITED TOO LONG FOR A ROW CACHE ENQUEUE LOCK! pid=45
System State dumped to trace file /opt/oracle/admin/dba/udump/mega_ora_23781.trc
Sat Jun 06 02:50:03 CST 2015
>>> WAITED TOO LONG FOR A ROW CACHE ENQUEUE LOCK! pid=46
System State dumped to trace file /opt/oracle/admin/dba/udump/mega_ora_30833.trc
Sat Jun 06 23:50:03 CST 2015
>>> WAITED TOO LONG FOR A ROW CACHE ENQUEUE LOCK! pid=47
System State dumped to trace file /opt/oracle/admin/dba/udump/mega_ora_4951.trc
Sun Jun 07 02:50:02 CST 2015
>>> WAITED TOO LONG FOR A ROW CACHE ENQUEUE LOCK! pid=48
System State dumped to trace file /opt/oracle/admin/dba/udump/mega_ora_5891.trc
Mon Jun 08 02:50:02 CST 2015
>>> WAITED TOO LONG FOR A ROW CACHE ENQUEUE LOCK! pid=49
System State dumped to trace file /opt/oracle/admin/dba/udump/mega_ora_7520.trc
Tue Jun 09 02:50:02 CST 2015
>>> WAITED TOO LONG FOR A ROW CACHE ENQUEUE LOCK! pid=50
System State dumped to trace file /opt/oracle/admin/dba/udump/mega_ora_14768.trc
Wed Jun 10 02:50:02 CST 2015
>>> WAITED TOO LONG FOR A ROW CACHE ENQUEUE LOCK! pid=51
System State dumped to trace file /opt/oracle/admin/dba/udump/mega_ora_23975.trc
Wed Jun 10 22:24:05 CST 2015
>>> WAITED TOO LONG FOR A ROW CACHE ENQUEUE LOCK! pid=52
System State dumped to trace file /opt/oracle/admin/dba/udump/mega_ora_29755.trc
Wed Jun 10 22:25:03 CST 2015
>>> WAITED TOO LONG FOR A ROW CACHE ENQUEUE LOCK! pid=53
System State dumped to trace file /opt/oracle/admin/dba/udump/mega_ora_29760.trc
Thu Jun 11 02:50:02 CST 2015
>>> WAITED TOO LONG FOR A ROW CACHE ENQUEUE LOCK! pid=54
System State dumped to trace file /opt/oracle/admin/dba/udump/mega_ora_31040.trc

2.在TRACE文件中包含一个SYSTEMSTATE DUMP ,首先通过ASS.awk来分析一下systemstate dump:
                     Resource Holder State Enqueue PR-00000000-00000000    41: last wait for 'os thread startup'       Latch c0000000c2df3b70    ??? Blocker Rcache object=c000000f9fdf8160,    61: waiting for 'log file switch (checkpoint incomplete)' Enqueue US-0000004C-00000000   185: waiting for 'log file switch (checkpoint incomplete)' Enqueue FB-00000006-0186BDC8   187: waiting for 'log file switch (checkpoint incomplete)' LOCK: handle=c000000f388db3d0   204: waiting for 'log file switch (checkpoint incomplete)'
Object Names ~~~~~~~~~~~~ Enqueue PR-00000000-00000000                               Latch c0000000c2df3b70       holding    (efd=5) c0000000c2df3b70 slave cl Rcache object=c000000f9fdf8160,                               Enqueue US-0000004C-00000000                               Enqueue FB-00000006-0186BDC8                               LOCK: handle=c000000f388db3d0 TABL:REPORT.STATQ_AGENT_SUBS_NEW 
看样子ORAPID=41的会话可能是BLOCKER，从ass生成的报告来看，系统中存在大量的log file switch（checkpoint incomplete）等待。我们首先查看一下latch c0000000c2df3b70的持有者：
PROCESS 41:   ----------------------------------------   SO: c000000fbe3c1910, type: 2, owner: 0000000000000000, flag: INIT/-/-/0x00   (process) Oracle pid=41, calls cur/top: c000000f9d63cbd0/c000000fa2c63088, flag: (2) SYSTEM             int error: 0, call error: 0, sess error: 0, txn error 0   (post info) last post received: 0 0 33               last post received-location: ksrpublish               last process to post me: c000000fba3f0d80 1 6               last post sent: 0 0 24               last post sent-location: ksasnd               last process posted by me: c000000fb93cbc00 1 6     (latch info) wait_event=0 bits=1       holding    (efd=5) c0000000c2df3b70 slave class create level=0         Location from where latch is held: ksvcreate:         Context saved from call: 0         state=busy, wlstate=free           waiters [orapid (seconds since: put on list, posted, alive check)]:            48 (61788, 1251079676, 3)            118 (61728, 1251079676, 0)            waiter count=2     Process Group: DEFAULT, pseudo proc: c000000fbc44e418     O/S info: user: oracle, term: UNKNOWN, ospid: 16929     OSD pid info: Unix process pid: 16929, image: <A href="mailto:oracle@qdbb1">oracle@qdbb1</A> (MMON) 
  mmon在等待：
    service name: SYS$BACKGROUND     last wait for 'os thread startup' blocking sess=0x0000000000000000 seq=59956 wait_time=1006468 seconds since wait started=62472                 =0, =0, =0     Dumping Session Wait History      for 'os thread startup' count=1 wait_time=1006468                 =0, =0, =0      for 'os thread startup' count=1 wait_time=958707                 =0, =0, =0      for 'os thread startup' count=1 wait_time=1017506                 =0, =0, =0      for 'os thread startup' count=1 wait_time=975671                 =0, =0, =0      for 'os thread startup' count=1 wait_time=976846                 =0, =0, =0      for 'os thread startup' count=1 wait_time=984219                 =0, =0, =0      for 'os thread startup' count=1 wait_time=585799                 =0, =0, =0      for 'os thread startup' count=1 wait_time=1355858                 =0, =0, =0      for 'os thread startup' count=1 wait_time=979583                 =0, =0, =0      for 'os thread startup' count=1 wait_time=990744                 =0, =0, =0 看样子mmon在启动其他进程的时候在等待启动结束:
    SO: c000000f9e6e3a20, type: 16, owner: c000000fbe3c1910, flag: INIT/-/-/0x00     (osp req holder)     CHILD REQUESTS:        (osp req) type=2(BACKGROUND) flags=0x20001(STATIC/-) state=8(DEAD) err=0                  pg=129(default) arg1=0 arg2=(null) reply=(null) pname=m000                  pid=0 parent=c000000f9e6e3a20 fulfill=0000000000000000 原来mmon在等待m000启动.那么mmon等待m000会导致整个系统HANG住吗？我们需要进一步分析

3.我们回过头来在来看这个>>> WAITED TOO LONG FOR A ROW CACHE ENQUEUE LOCK! pid=XXX
这个等待从临晨4点多到宕机前一直存在。通过ass的结果，我们可以看到：
254:waiting for 'row cache lock'       [Rcache object=c000000f9fdf8160,] wait 255:waiting for 'row cache lock'       [Rcache object=c000000f9fdf8160,] wait 256:waiting for 'row cache lock'       [Rcache object=c000000f9fdf8160,] wait 257:waiting for 'row cache lock'       [Rcache object=c000000f9fdf8160,] wait 258:waiting for 'row cache lock'       [Rcache object=c000000f9fdf8160,] wait 259:waiting for 'row cache lock'       [Rcache object=c000000f9fdf8160,] wait 260:waiting for 'row cache lock'       [Rcache object=c000000f9fdf8160,] wait 261:waiting for 'row cache lock'       [Rcache object=c000000f9fdf8160,] wait 262:waiting for 'row cache lock'       [Rcache object=c000000f9fdf8160,] wait 263:waiting for 'row cache lock'       [Rcache object=c000000f9fdf8160,] wait 264:waiting for 'row cache lock'       [Rcache object=c000000f9fdf8160,] wait 265:waiting for 'row cache lock'       [Rcache object=c000000f9fdf8160,] wait 266:waiting for 'row cache lock'       [Rcache object=c000000f9fdf8160,] wait 267:waiting for 'row cache lock'       [Rcache object=c000000f9fdf8160,] wait 268:waiting for 'row cache lock'       [Rcache object=c000000f9fdf8160,] wait 269:waiting for 'row cache lock'       [Rcache object=c000000f9fdf8160,] wait 270:waiting for 'row cache lock'       [Rcache object=c000000f9fdf8160,] wait 271:waiting for 'row cache lock'       [Rcache object=c000000f9fdf8160,] wait 272:waiting for 'row cache lock'       [Rcache object=c000000f9fdf8160,] wait 273:waiting for 'row cache lock'       [Rcache object=c000000f9fdf8160,] wait 274:waiting for 'row cache lock'       [Rcache object=c000000f9fdf8160,] wait 275:waiting for 'row cache lock'       [Rcache object=c000000f9fdf8160,] wait 276:waiting for 'row cache lock'       [Rcache object=c000000f9fdf8160,] wait 277:waiting for 'row cache lock'       [Rcache object=c000000f9fdf8160,] wait 278:waiting for 'row cache lock'       [Rcache object=c000000f9fdf8160,] wait 279:waiting for 'row cache lock'       [Rcache object=c000000f9fdf8160,] wait 280:waiting for 'row cache lock'       [Rcache object=c000000f9fdf8160,] wait 281:waiting for 'row cache lock'       [Rcache object=c000000f9fdf8160,] wait 
检查一下process 254
    service name: SYS$USERS     O/S info: user: report, term: , ospid: 17444, machine: qdbb2               program: <A href="mailto:task@qdbb2">task@qdbb2</A> (TNS V1-V3)     application name: <A href="mailto:task@qdbb2">task@qdbb2</A> (TNS V1-V3), hash value=516955253     waiting for 'row cache lock' blocking sess=0x0000000000000000 seq=1 wait_time=0 seconds since wait started=3425                 cache id=7, mode=0, request=3     Dumping Session Wait History      for 'row cache lock' count=1 wait_time=2615950                 cache id=7, mode=0, request=3      for 'row cache lock' count=1 wait_time=2929664                 cache id=7, mode=0, request=3      for 'row cache lock' count=1 wait_time=2929674                 cache id=7, mode=0, request=3      for 'row cache lock' count=1 wait_time=2929666                 cache id=7, mode=0, request=3 
      SO: c000000f9f9198b8, type: 50, owner: c000000ee30a3060, flag: INIT/-/-/0x00       row cache enqueue: count=1 session=c000000fba59b660 object=c000000f9fdf8160, request=S       savepoint=0x4       row cache parent object: address=c000000f9fdf8160 cid=7(dc_users)       hash=cdb2e447 typ=11 transaction=c000000faae8c710 flags=0000012a       own=c000000f9fdf8230[c000000f9f919a08,c000000f9f919a08] wat=c000000f9fdf8240[c000000f9f9198e8,c000000f9f918b08] mode=X
这是一个dc_users的对象，请求s方式的访问，目前在等待一个mode=x的等待释放，因此我们搜索c000000f9fdf8160, mode=X，找到：
        SO: c000000f9f9199d8, type: 50, owner: c000000faae8c710, flag: INIT/-/-/0x00         row cache enqueue: count=1 session=c000000fba5f3b30 object=c000000f9fdf8160, mode=X         savepoint=0x6         row cache parent object: address=c000000f9fdf8160 cid=7(dc_users)         hash=cdb2e447 typ=11 transaction=c000000faae8c710 flags=0000012a         own=c000000f9fdf8230[c000000f9f919a08,c000000f9f919a08] wat=c000000f9fdf8240[c000000f9f9198e8,c000000f9f918b08] mode=X         status=VALID/UPDATE/-/-/IO/-/-/-/-         request=N release=TRUE flags=0         instance lock id=QH b6ae8f92 4cc6da8c         set=0, complete=FALSE         set=1, complete=FALSE         set=2, complete=FALSE 就是这个会话持有了这个dc_users的row cache object，而且持有方式是不兼容的，往前看，发现这个会话是：
PROCESS 61:   ----------------------------------------   SO: c000000fbf3bfe48, type: 2, owner: 0000000000000000, flag: INIT/-/-/0x00   (process) Oracle pid=61, calls cur/top: c000000fa0c0eb30/c000000f8b0fdef8, flag: (0) -             int error: 0, call error: 0, sess error: 0, txn error 0   (post info) last post received: 109 0 4               last post received-location: kslpsr               last process to post me: c000000fbc3c2d98 1 6               last post sent: 0 0 24               last post sent-location: ksasnd               last process posted by me: c000000fbc3c2d98 1 6     (latch info) wait_event=109 bits=0     Process Group: DEFAULT, pseudo proc: c000000fbc44e418     O/S info: user: oracle, term: UNKNOWN, ospid: 2006     OSD pid info: Unix process pid: 2006, image: <A href="mailto:oracle@qdbb1">oracle@qdbb1</A> 
    SO: c000000fba5f3b30, type: 4, owner: c000000fbf3bfe48, flag: INIT/-/-/0x00     (session) sid: 1973 trans: c000000faae8c710, creator: c000000fbf3bfe48, flag: (41) USR/- BSY/-/-/-/-/-               DID: 0001-003D-041A6D36, short-term DID: 0000-0000-00000000               txn branch: 0000000000000000               oct: 0, prv: 0, sql: 0000000000000000, psql: 0000000000000000, user: 0/SYS     service name: SYS$USERS     O/S info: user: liping, term: L902054, ospid: 163000:163128, machine:               program: plsqldev.exe     application name: plsqldev.exe, hash value=2126984564     waiting for 'log file switch (checkpoint incomplete)' blocking sess=0x0000000000000000 seq=9 wait_time=0 seconds since wait started=2290                 =0, =0, =0     Dumping Session Wait History      for 'log file switch (checkpoint incomplete)' count=1 wait_time=87117                 =0, =0, =0      for 'log file switch (checkpoint incomplete)' count=1 wait_time=976593                 =0, =0, =0      for 'log file switch (checkpoint incomplete)' count=1 wait_time=979179                 =0, =0, =0      for 'log file switch (checkpoint incomplete)' count=1 wait_time=7099                 =0, =0, =0      for 'log file switch (checkpoint incomplete)' count=1 wait_time=976471                 =0, =0, =0      for 'log file switch (checkpoint incomplete)' count=1 wait_time=976611                 =0, =0, =0      for 'log file switch (checkpoint incomplete)' count=1 wait_time=973437                 =0, =0, =0      for 'log file switch (checkpoint incomplete)' count=1 wait_time=969860                 =0, =0, =0      for 'log file switch (checkpoint incomplete)' count=1 wait_time=976535                 =0, =0, =0      for 'log file switch (checkpoint incomplete)' count=1 wait_time=978815                 =0, =0, =0 这是一个PL/SQL DEVELOPER的用户，不过从上面的情况看，这个会话是产生了阻塞大量会话的源头，不过这个会话本身也是受害者，在等待log file switch(checkpoint incomplete),为什么会这样呢？

4.于是我们来看看lgwr的状态：
    SO: c000000fba6167c0, type: 4, owner: c000000fbc3c2d98, flag: INIT/-/-/0x00     (session) sid: 2155 trans: 0000000000000000, creator: c000000fbc3c2d98, flag: (51) USR/- BSY/-/-/-/-/-               DID: 0001-0024-00000005, short-term DID: 0000-0000-00000000               txn branch: 0000000000000000               oct: 0, prv: 0, sql: 0000000000000000, psql: 0000000000000000, user: 0/SYS     service name: SYS$BACKGROUND     waiting for 'rdbms ipc message' blocking sess=0x0000000000000000 seq=13185 wait_time=0 seconds since wait started=0                 timeout=d3, =0, =0     Dumping Session Wait History      for 'control file sequential read' count=1 wait_time=233                 file#=0, block#=2e, blocks=1      for 'control file sequential read' count=1 wait_time=210                 file#=0, block#=2c, blocks=1      for 'control file sequential read' count=1 wait_time=255                 file#=0, block#=29, blocks=1      for 'control file sequential read' count=1 wait_time=213                 file#=0, block#=27, blocks=1      for 'control file sequential read' count=1 wait_time=222                 file#=2, block#=1, blocks=1      for 'control file sequential read' count=1 wait_time=221                 file#=1, block#=1, blocks=1      for 'control file sequential read' count=1 wait_time=282                 file#=0, block#=1, blocks=1      for 'rdbms ipc message' count=1 wait_time=4564                 timeout=d4, =0, =0      for 'rdbms ipc message' count=1 wait_time=3188                 timeout=d4, =0, =0      for 'rdbms ipc message' count=1 wait_time=1                 timeout=d4, =0, =0
LGWR居然在等待ipc message，说明lgwr并没有处于工作状态，或者无事可做，或者等待其他会话处理结束后唤醒它。而目前大家都在等待log file switch，lgwr没有闲着没事做的理由，因此我们有理由相信checkpoint出问题了，lgwr是在等待ckpt完成工作，于是我们继续检查ckpt的状态：

5.搜索(ckpt)
    SO: c000000fbb632130, type: 4, owner: c000000fbb40bc08, flag: INIT/-/-/0x00     (session) sid: 2154 trans: 0000000000000000, creator: c000000fbb40bc08, flag: (51) USR/- BSY/-/-/-/-/-               DID: 0001-0025-00000004, short-term DID: 0001-0025-00000005               txn branch: 0000000000000000               oct: 0, prv: 0, sql: 0000000000000000, psql: 0000000000000000, user: 0/SYS     service name: SYS$BACKGROUND     waiting for 'enq: PR - contention' blocking sess=0xc000000fbf5e4278 seq=61352 wait_time=0 seconds since wait started=62446                 name|mode=50520006, 0=0, 0=0     Dumping Session Wait History      for 'enq: PR - contention' count=1 wait_time=429069                 name|mode=50520006, 0=0, 0=0
     for 'enq: PR - contention' count=1 wait_time=2929688                 name|mode=50520006, 0=0, 0=0      for 'enq: PR - contention' count=1 wait_time=2929668                 name|mode=50520006, 0=0, 0=0      for 'enq: PR - contention' count=1 wait_time=2929682                 name|mode=50520006, 0=0, 0=0      for 'enq: PR - contention' count=1 wait_time=2929671                 name|mode=50520006, 0=0, 0=0      for 'enq: PR - contention' count=1 wait_time=2929679                 name|mode=50520006, 0=0, 0=0      for 'enq: PR - contention' count=1 wait_time=2929666                 name|mode=50520006, 0=0, 0=0      for 'enq: PR - contention' count=1 wait_time=2929683                 name|mode=50520006, 0=0, 0=0      for 'enq: PR - contention' count=1 wait_time=2929624                 name|mode=50520006, 0=0, 0=0      for 'enq: PR - contention' count=1 wait_time=2929676                 name|mode=50520006, 0=0, 0=0 ckpt一直在等待PR锁，这回我们终于看到了blocking sess=0xc000000fbf5e4278 ，这个会话就是阻塞ckpt的元凶，我们来看看是谁：
    SO: c000000fbf5e4278, type: 4, owner: c000000fbe3c1910, flag: INIT/-/-/0x00     (session) sid: 2150 trans: 0000000000000000, creator: c000000fbe3c1910, flag: (100051) USR/- BSY/-/-/-/-/-               DID: 0001-0029-00000002, short-term DID: 0001-0029-00000003               txn branch: 0000000000000000               oct: 0, prv: 0, sql: c000000fa3ed29b8, psql: 0000000000000000, user: 0/SYS     service name: SYS$BACKGROUND     last wait for 'os thread startup' blocking sess=0x0000000000000000 seq=59956 wait_time=1006468 seconds since wait started=62472                 =0, =0, =0 我们又找到了mmon，原来mmon才是导致系统HANG住的元凶。

6.案例总结：
1、到目前为止，我们弄清楚了系统HANG住的主要原因是MMON启动m000的时候，m000没能够正常启动，导致mmon长期持有pr锁，阻塞了ckpt，导致无法正常做日志切换，但是mmon未能正常启动m000的原因还没有找到，这需要更多的资料进行分析，不在本文讨论之内
2、碰到这种问题如何处理？实际上如果我们分析清楚了，只要杀掉mmon进程，或者调整一下一下statistics_level就可能解决问题，而不需要重启实例。但是这个分析需要时间，最起码也需要30-60分钟的分析时间，如果在生产环境下发生，很难给我们这么长的分析时间。但是大家要注意这个问题的出现时在4点多钟，系统HANG住是在10点多钟，中间有很长时间供我们分析。如果我们能及时发现这个异常，是可能在系统HANG住钱解决问题的

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WAITED TOO LONG FOR A ROW CACHE ENQUEUE LOCK!的分析

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