Oracle故障处理之错误代码：Warning: VKTM detected a time drift.

数据与人 2020-12-15

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问题背景：

客户反馈alert日志中大量告警

告警如下：

Warning: VKTM detected a time drift.
Time drifts can result in an unexpected behavior such as time-outs. Please check trace file for more details.
Tue Dec 03 16:21:35 2019
Warning: VKTM detected a time drift.
Time drifts can result in an unexpected behavior such as time-outs. Please check trace file for more details.
Tue Dec 03 16:38:23 2019
Adjusting the default value of parameter parallel_max_servers
from 640 to 320 due to the value of parameter processes (350)
Starting ORACLE instance (normal)
Tue Dec 03 21:09:32 2019
Warning: VKTM detected a time drift.
Time drifts can result in an unexpected behavior such as time-outs. Please check trace file for more details.
Tue Dec 03 22:00:00 2019
Setting Resource Manager plan SCHEDULER[0x32DA]:DEFAULT_MAINTENANCE_PLAN via scheduler window
Setting Resource Manager plan DEFAULT_MAINTENANCE_PLAN via parameter
Tue Dec 03 22:00:00 2019
Starting background process VKRM
Tue Dec 03 22:00:00 2019
VKRM started with pid=78, OS id=28922 
Tue Dec 03 22:00:02 2019
Begin automatic SQL Tuning Advisor run for special tuning task "SYS_AUTO_SQL_TUNING_TASK"
Tue Dec 03 22:20:55 2019
End automatic SQL Tuning Advisor run for special tuning task "SYS_AUTO_SQL_TUNING_TASK"
Tue Dec 03 22:46:17 2019
Warning: VKTM detected a time drift.
Time drifts can result in an unexpected behavior such as time-outs. Please check trace file for more details.
Wed Dec 04 00:00:29 2019
Thread 1 advanced to log sequence 5843 (LGWR switch)
在Oracle Database 11g中，VKTM是一个新增的后台进程。这个进程的含义是：
VKTM (virtual keeper of time) is responsible for providing a wall-clock time (updated every second) and 
reference-time counter (updated every 20 ms and available only when running at elevated priority).

也就是说，这个进程用于提供一个数据库的时钟，每秒更新；或者作为参考时间计数器，这种方式每20毫秒更新一次，仅在高优先级时可用。

在系统时间出现异常或变化时，VKTM进程还会检测这些变化，提醒用户，尤其是在RAC环境中，时间的偏移和变化极有可能导致系统故障。以下是告警日志中的警告信息：

Warning: VKTM detected a time drift.
Time drifts can result in an unexpected behavior such as time-outs. Please check trace file for more details.

通过VKTM进程，数据库可以降低和操作系统的交互，Kamus的解释是：

在11g之前所有的Oracle数据库后台或者前台进程如果需要获得当前时间信息，就需要调用操作系统的gettimeofday()函数或者说是相类似的函数。
而VKTM进程就是专门用来获得时间信息然后将信息存放在SGA中供其它进程使用，这样其它进程当需要时间信息的时候，只要到SGA的某个内存位置去获得就好，
而不用频繁调用gettimeofday()函数。毫无疑问，这样效率会更高。

在RAC测试中，Oracle 11.1.0.6版本LMSx进程获取时间信息时，可以从VKTM进程中获益大概70%的速度提升，而11.1.0.7将会更高。

同时，因为gettimeofday()函数也引发了很多bug，所以无论是RAC还是NORAC库，都将从VKTM进程中获益。

往期回顾

Oracle故障处理之ORA-00600: error, arguments: [kcbo_unlink_q_3]

Oracle故障处理之impdp导入报错：ORA-39126

Oracle故障处理之ORA-27102: out of memory

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Oracle故障处理之错误代码：Warning: VKTM detected a time drift.

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