[20161228]奇怪log file sync等待事件.txt
--这个来自链接:http://www.itpub.net/thread-2073857-1-1.html的讨论,很奇怪的问题:
Top 10 Foreground Events by Total Wait Time
Event Waits Total Wait Time (sec) Wait Avg(ms) % DB time Wait Class
log file sync 47,412 50K 1054 68.2 Commit
DB CPU 7250.1 9.9
log file switch (private strand flush incomplete) 420 5179.7 12333 7.1 Configuration
enq: TX - row lock contention 46 4448.1 96698 6.1 application
JS kgl get object wait 3,814 1366 358 1.9 Administrative
JS kill job wait 1,269 1278.1 1007 1.7 Administrative
SQL*Net message from dblink 286,374 602.3 2 .8 Network
direct path write temp 30,149 405.4 13 .6 User I/O
buffer busy waits 302 376.2 1246 .5 Concurrency
db file sequential read 49,254 301.4 6 .4 User I/O
--30分钟awr报表,log file sync 达到了50K sec,一般出现这个多数情况是存放redo磁盘IO很慢,或者事务非常多.硬件不堪重负.
--而仔细看db file sequential read 平均等待事件仅仅6ms,而且lz介绍数据文件与redo在同一个磁盘组.
--而看前面Load Profile
Load Profile
Per Second Per Transaction Per Exec Per Call
DB Time(s): 36.7 1.4 0.04 0.02
DB CPU(s): 3.6 0.1 0.00 0.00
Redo size (bytes): 164,938.0 6,263.7
--平均每秒的redo size仅仅160K.这样的量对于现在的磁盘轻松应付.而再仔细看Background Wait Events:
Background Wait Events
ordered by wait time desc, waits desc (idle events last)
Only events with Total Wait Time (s) >= .001 are shown
%Timeouts: value of 0 indicates value was < .5%. Value of null is truly 0
Event Waits %Time -outs Total Wait Time (s) Avg wait (ms) Waits /txn % bg time
LNS wait on SENDREQ 21,345 0 1,826 86 0.41 37.53
LGWR-LNS wait on channel 116,328 93 1,647 14 2.21 33.86
log file parallel write 13,220 0 263 20 0.25 5.40
--//前面2者相加,占用大约70%的后台时间.而这个数值几乎与log file sync占用的db time 68.2% ,相互呼应.
--而dg的参数如下:
log_archive_dest_2 service="XXXX1", LGWR SYNC AFFIRM delay=0 optional compression=disable max_failure=0
max_connections=1 reopen=300 db_unique_name="XXXX1" net_timeout=30, valid_for=(all_logfiles, primary_role)
SERVICE="XXXX1" LGWR ASYNC valid_for=(all_logfiles, primary_role) DB_UNIQUE_NAME="XXXX1"
--//前面是sync,后面是async,估计应该sync.可以推测这个设置导致出现log file sync.
--//检查oracle相关文档:http://docs.oracle.com/cd/E11882_01/server.112/e41134/log_arch_dest_param.htm#SBYDB01114
SYNC and ASYNC
Specifies whether the synchronous (SYNC) or asynchronous (ASYNC) redo transport mode is to be used.
Usage Notes
The LOG_ARCHIVE_DEST_11 through LOG_ARCHIVE_DEST_31 parameters do not support the SYNC attribute.
The redo data generated by a transaction must have been received by every enabled destination that has the SYNC
attribute before that transaction can commit.
The redo data generated by a transaction need not have been received at a destination that has the ASYNC attribute
before that transaction can commit. This is the default behavior if neither SYNC or ASYNC is specified.
--//英文我就不翻译了,我估计应该是对方的网络问题导致这个问题,目前仅仅是猜测,等待对方的确认.
