4.12 Remember Daylight Savings Time
4.12.1 Description
Daylight savings time has the potential of wreaking havoc on your
system. Everyone always thinks of the time change and checks all the system
clocks, but what about the crontab?
Reason
You always remember during these two important days of the year
to verify that all the system clocks are current on all the systems in your
environment. But did you remember to check the jobs that were scheduled to run
in the crontab? Any job that is set up to run between 1:00 a.m. and 2:59 a.m.
is affected by daylight savings time. This isn't once every 100 years or
even once a year.
Real World Experience
In the month of October, all the entries in the crontab that are
scheduled to run from 1:00 a.m. to 1:59 a.m. run twice. Many quick and dirty
scripted programs that have been written in a hurry cannot handle such a situation.
Most commercial programs written take into the account that a user (even us)
might execute the program twice.
In most cases you might not have anything too serious scheduled
during this time period. If you do have jobs scheduled between these periods
ask yourself what happens...
-
When the jobs run twice?
- When the jobs take over an hour to finish and the same job
executes and overlaps with the previous job?
- If there are any open files? Will they get corrupted?
- When the database performs an update at that time?
- To any system files that are touched or are affected during
that time?
- To user-scheduled jobs?
When daylight savings time ends in the month of April, anything
scheduled between 2:00 a.m. and 2:59 a.m. in the crontab do not get executed
at all. How can this affect your environment? Many scheduled jobs are required
to run for your users to be able to function during their next working day.
What happens when their jobs don't run? Will you be ready?
You need to look at your crontab entries on all your systems.
Make your users aware of what can happen during these time periods. Depending
on the trust and danger factor of your users and their levels of UNIX experience,
you might want to examine their crontab entries to see what jobs they are running.
The key word here is examine. You have the ability and just cause to
make sure your systems are not threatened or abused by any user. Management
should back you up on this. If you feel in your environment that you need management
approval, discuss your concerns with them.
Some
sites don't use cron that much. If your site is one of these, you might
want to schedule jobs outside this area of time so you are not affected.
Other Resources
Man pages:
at, cron,
crontab
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