According to MySQL Documentation, the global variable Uptime is defined as "The number of seconds that the server has been up.".
However, can somebody please explain to me how this value is actually computed? What does it use as a reference, System Time?
I am asking this question because I just came across a weird situation : when rebooting a VM with MySQL, ntpd service terminated, and at startup (since was not on chkconfig), the time got shifted +8 hours, as you can see by the following :
15:01:00 hostname shutdown[30383]: shutting down for system reboot
15:01:00 hostname init: Switching to runlevel: 6
...
15:01:06 hostname ntpd[27553]: ntpd exiting on signal 15
15:01:06 hostname syslog-ng[27399]: Termination requested via signal, terminating;
...
23:04:03 hostname kernel: Bootdata ok
The same shift is recorded in the MySQL error logs :
15:01:03 InnoDB: Starting shutdown...
15:01:05 InnoDB: Shutdown completed; log sequence number 2746293826
15:01:06 [Note] /usr/sbin/mysqld: Shutdown complete
15:01:06 mysqld_safe mysqld from pid file /var/lib/mysql/data/hostname.pid ended
23:04:06 mysqld_safe Starting mysqld daemon with databases from /var/lib/mysql/data
After we fixed the time by starting ntpd, it seemed that the Uptime got shifted :
mysql> show global status like 'Uptime';
+---------------+-------+
| Variable_name | Value |
+---------------+-------+
| Uptime | 18005 |
+---------------+-------+
1 row in set (0.00 sec)
mysql> show global status like 'Uptime_since_flush_status';
+---------------------------+-------+
| Variable_name | Value |
+---------------------------+-------+
| Uptime_since_flush_status | 18007 |
+---------------------------+-------+
Is this behavior possible, or it probably related to other factors?
Thank you for your patience and understanding.
Should be very simple. The application will create a timestamp from when it starts and compare it to the current time. These times are given from the system time.
So if you modify the system time, it will not adjust the initial timestamp. It will consider the time change as the current time and relay that as its comparison.