log file sync等待超高案例浅析

监控工具DPA发现海外一台Oracle数据库服务器DB Commit Time指标告警，超过红色告警线（40毫秒左右，黄色告警是10毫秒，红色告警线是20毫秒），如下截图所示，生成了对应的时段的AWR报告，发现Top 5 Timed Events里面，log file sync等待事件的平均等待时间为37毫秒，log file parallel write等待事件的平均等待时间为40毫秒

 

 

 

 

如果对Tanel Poder：:Understanding LGWR, Log File Sync Waits and Commit Performance这篇文章所讲述的内容很熟悉的话（经典图如下），那么通过等待事件log file sync与log file parallel write的Avg Wait(ms）指标： 37ms & 40ms,基本上可以判断就是redo log所在的磁盘I/O出现了性能问题

 

 

 

 

然后在用lfsdiag.sql脚本分析一下详细的统计数据，如下所示：

 

 

 

 

HISTOGRAM DATA FOR LFS AND OTHER RELATED WAITS:

APPROACH: Look at the wait distribution for log file sync waits
by looking at "wait_time_milli". Look at the high wait times then
see if you can correlate those with other related wait events.

   INST_ID EVENT                                    WAIT_TIME_MILLI WAIT_COUNT
---------- ---------------------------------------- --------------- ----------
         1 log file sync                                          1       4363
         1 log file sync                                          2        835
         1 log file sync                                          4       1650
         1 log file sync                                          8       4937
         1 log file sync                                         16     146252
         1 log file sync                                         32     606674
         1 log file sync                                         64     263377
         1 log file sync                                        128     253254
         1 log file sync                                        256          2
         1 log file switch completion                             1        124
         1 log file switch completion                             2          9
         1 log file switch completion                             4         19
         1 log file switch completion                             8         21
         1 log file switch completion                            16         35
         1 log file switch completion                            32         97
         1 log file switch completion                            64        133
         1 log file switch completion                           128        326
         1 log file switch completion                           256       1736
         1 log file switch completion                           512       3042
         1 log file switch completion                          1024       2020
         1 log file parallel write                                1          0
         1 log file parallel write                                2          0
         1 log file parallel write                                4         80
         1 log file parallel write                                8       2142
         1 log file parallel write                               16     170987
         1 log file parallel write                               32     779205
         1 log file parallel write                               64     311463
         1 log file parallel write                              128      79688
         1 log file parallel write                              256      42763
         1 log file parallel write                              512      13052
         1 log file parallel write                             1024      20468
         1 log file parallel write                             2048      14020
         1 log file parallel write                             4096        921
         1 log file parallel write                             8192         96
         1 log file parallel write                            16384         18
         1 log file parallel write                            32768         18
         1 log file parallel write                            65536          8
         1 log file parallel write                           131072          2
         1 LGWR wait for redo copy                                1       8516
         1 LGWR wait for redo copy                                2         20
         1 LGWR wait for redo copy                                4         19
         1 LGWR wait for redo copy                                8         20
         1 LGWR wait for redo copy                               16         11

ORDERED BY WAIT_TIME_MILLI

   INST_ID EVENT                                    WAIT_TIME_MILLI WAIT_COUNT
---------- ---------------------------------------- --------------- ----------
         1 log file sync                                          1       4363
         1 log file switch completion                             1        124
         1 log file parallel write                                1          0
         1 LGWR wait for redo copy                                1       8516
         1 log file sync                                          2        835
         1 log file switch completion                             2          9
         1 log file parallel write                                2          0
         1 LGWR wait for redo copy                                2         20
         1 log file sync                                          4       1650
         1 log file switch completion                             4         19
         1 log file parallel write                                4         80
         1 LGWR wait for redo copy                                4         19
         1 log file sync                                          8       4937
         1 log file switch completion                             8         21
         1 log file parallel write                                8       2142
         1 LGWR wait for redo copy                                8         20
         1 log file sync                                         16     146252
         1 log file switch completion                            16         35
         1 log file parallel write                               16     170987
         1 LGWR wait for redo copy                               16         11
         1 log file sync                                         32     606674
         1 log file switch completion                            32         97
         1 log file parallel write                               32     779205
         1 log file sync                                         64     263377
         1 log file switch completion                            64        133
         1 log file parallel write                               64     311463
         1 log file sync                                        128     253254
         1 log file switch completion                           128        326
         1 log file parallel write                              128      79688
         1 log file sync                                        256          2
         1 log file switch completion                           256       1736
         1 log file parallel write                              256      42763
         1 log file switch completion                           512       3042
         1 log file parallel write                              512      13052
         1 log file switch completion                          1024       2020
         1 log file parallel write                             1024      20468
         1 log file parallel write                             2048      14020
         1 log file parallel write                             4096        921
         1 log file parallel write                             8192         96
         1 log file parallel write                            16384         18
         1 log file parallel write                            32768         18
         1 log file parallel write                            65536          8
         1 log file parallel write                           131072          2

REDO WRITE STATS

"redo write time" in centiseconds (100 per second)
11.1: "redo write broadcast ack time" in centiseconds (100 per second)
11.2: "redo write broadcast ack time" in microseconds (1000 per millisecond)

VERSION              INST_ID NAME                                                     VALUE        MILLISECONDS
----------------- ---------- ---------------------------------------- --------------------- -------------------
10.2.0.5.0                 1 redo write time                                        9551524        95515240.000
10.2.0.5.0                 1 redo writer latching time                                   51
10.2.0.5.0                 1 redo writes                                            1434931

AWR WORST AVG LOG FILE SYNC SNAPS:

 

上面数据可以看到，log file sync等待事件数量最多的是32ms这个区间的，log file parallel write等待事件发生最多的也是32ms这个区间的，其实这个值已经远远超过7ms，极其不正常。log file parallel write 事件是LGWR进程专属的等待事件，发生在LGWR将log_buffer中的重做日志信息写入联机重做日志文件组的成员文件，LGWR在该事件上等待该写入过程的完成。该事件等待时间过长，说明日志文件所在磁盘缓慢或存在争用。log file sync和log file parallel write是相互关联的。换句话讲，假设log file parallel write的时间非常长，那么必定导致log file sync等待时间拉长。如果log file parallel write 等待非常高，那么可能一般是物理磁盘I/O的问题

 

另外，我们也检查了一下redo log的切换频率，如下所示，redolog_sitch_time.sql查看发现redo log切换的次数并不频繁，生成的归档日志的量也并不大。大部分时候一小时切换零次或一次。

 

 

 

然后我们找了一台机器(上述指标正常的服务器)简单对测试了一下IO的速度，这个方法极其简单，就是看看生成一个大文件需要多长时间，简单测试一下I/O性能（没有考虑cache等，测试采样也不详尽），但是对比数据也基本能验证、反馈磁盘IO存在问题）。

 

 

 

问题服务器：

 

 

# time dd if=/dev/zero of=./test bs=512k count=2048 oflag=direct

2048+0 records in

2048+0 records out

1073741824 bytes (1.1 GB) copied, 88.271 seconds, 12.2 MB/s

 

real    1m28.273s

user    0m0.010s

sys     0m0.655s

 

 

 

对比服务器（正常的服务器）：

 

 

 

 

# time dd if=/dev/zero of=./test bs=512k count=2048 oflag=direct

2048+0 records in

2048+0 records out

1073741824 bytes (1.1 GB) copied, 2.48344 seconds, 432 MB/s

 

real    0m2.485s

user    0m0.004s

sys     0m0.386s

 

 

  如上对比所示，两台服务器生成同样一个大小文件，耗费的时间，I/O性能差别非常大，完全验证了告警的服务器所在的存储I/O存在性能问题，但是公司分工非常明确，DBA也不清楚底层存储出了什么问题，只能将这个问题反馈出来，等待海外负责维护系统和存储的同事的回复。