1 Warning: VKTM detected a time drift. 2 Time drifts can result in an unexpected behavior such as time-outs. Please check trace file for more details. 3 Tue Dec 03 16:21:35 2019 4 Warning: VKTM detected a time drift. 5 Time drifts can result in an unexpected behavior such as time-outs. Please check trace file for more details. 6 Tue Dec 03 16:38:23 2019 7 Adjusting the default value of parameter parallel_max_servers 8 from 640 to 320 due to the value of parameter processes (350) 9 Starting ORACLE instance (normal) 10 Tue Dec 03 21:09:32 2019 11 Warning: VKTM detected a time drift. 12 Time drifts can result in an unexpected behavior such as time-outs. Please check trace file for more details. 13 Tue Dec 03 22:00:00 2019 14 Setting Resource Manager plan SCHEDULER[0x32DA]:DEFAULT_MAINTENANCE_PLAN via scheduler window 15 Setting Resource Manager plan DEFAULT_MAINTENANCE_PLAN via parameter 16 Tue Dec 03 22:00:00 2019 17 Starting background process VKRM 18 Tue Dec 03 22:00:00 2019 19 VKRM started with pid=78, OS id=28922 20 Tue Dec 03 22:00:02 2019 21 Begin automatic SQL Tuning Advisor run for special tuning task "SYS_AUTO_SQL_TUNING_TASK" 22 Tue Dec 03 22:20:55 2019 23 End automatic SQL Tuning Advisor run for special tuning task "SYS_AUTO_SQL_TUNING_TASK" 24 Tue Dec 03 22:46:17 2019 25 Warning: VKTM detected a time drift. 26 Time drifts can result in an unexpected behavior such as time-outs. Please check trace file for more details. 27 Wed Dec 04 00:00:29 2019 28 Thread 1 advanced to log sequence 5843 (LGWR switch)
在Oracle Database 11g中，VKTM是一個新增的后台進程。這個進程的含義是：
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).
也就是說，這個進程用於提供一個數據庫的時鍾，每秒更新；或者作為參考時間計數器，這種方式每20毫秒更新一次，僅在高優先級時可用。
在系統時間出現異常或變化時，VKTM進程還會檢測這些變化，提醒用戶，尤其是在RAC環境中，時間的偏移和變化極有可能導致系統故障。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.
通過VKTM進程，數據庫可以降低和操作系統的交互，Kamus的解釋是：
在11g之前所有的Oracle數據庫后台或者前台進程如果需要獲得當前時間信息，就需要調用操作系統的gettimeofday()函數或者說是相類似的函數。
而VKTM進程就是專門用來獲得時間信息然后將信息存放在SGA中供其它進程使用，這樣其它進程當需要時間信息的時候，只要到SGA的某個內存位置去獲得就好，
而不用頻繁調用gettimeofday()函數。毫無疑問，這樣效率會更高。
在RAC測試中，Oracle 11.1.0.6版本LMSx進程獲取時間信息時，可以從VKTM進程中獲益大概70%的速度提升，而11.1.0.7將會更高。
同時，因為gettimeofday()函數也引發了很多bug，所以無論是RAC還是NORAC庫，都將從VKTM進程中獲益。
參考
https://www.eygle.com/archives/2012/06/oracle_11g_vktm.html