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。
