[systemd-bugs] [Bug 77938] New: RFE: Allow customization of time and day of hourly, daily, weekly and monthly timers

[bugzilla-daemon at freedesktop.org]

https://bugs.freedesktop.org/show_bug.cgi?id=77938

          Priority: medium
            Bug ID: 77938
          Assignee: systemd-bugs at lists.freedesktop.org
           Summary: RFE: Allow customization of time and day of hourly,
                    daily, weekly and monthly timers
        QA Contact: systemd-bugs at lists.freedesktop.org
          Severity: enhancement
    Classification: Unclassified
                OS: Linux (All)
          Reporter: e2270733 at drdrb.com
          Hardware: All
            Status: NEW
           Version: unspecified
         Component: general
           Product: systemd

Timers with specify OnCalendar=hourly/daily/weekly/monthly currently assume 0
as parts of timestamp that are not specified. This results in timers elapsing
at a predefined fixed time (full hour / midnight / Monday). I propose to allow
for customization of that time, preferably by means of global settings. For
example, a setting ElapseDailyTimers="05:20:00" would change the meaning of
timestamp "daily" from "*-*-* 00:00:00" to "*-*-* 05:20:00". This would result
in daily timers being fired at 5:20am, rather than at midnight. Note that the
setting will be at a global level, not on a unit level. The name of the setting
is just an example.

Rationale:
Distributions are beginning to replace cron tasks by systemd timers, a recent
example being Arch Linux (please see [1]). The repository packages for various
system maintenance utilities are being modified to include timer files, and
these files typically specify OnCalendar timestamps as daily, weekly or
monhtly. This has an unexpected consequence of all maintenance tasks shifting
to midnight as opposed to early morning hours (4-6 am), as has been usual for
cron tasks. Midnight is not an ideal time to run maintenance tasks (too early).
In addition, in a multi-machine setup, this results in all machines performing
their maintenance at or nearly the same time. There is now no way to spread
these activities over a multiple hour period, as was possible by configuring
cron separately on each machine. Please see [2] for description of a particular
problem.
A similar argument can be given for weekly timers, for which Monday midnight
may not be a preferable time, as well as for hourly and monthly timers.

Discussion of available workarounds:
The only workaround within systemd framework is to override timer files and
replace OnCalendar timestamps with a specific time (e.g. "05:20" instead of
"daily"). This is unfortunately quite cumbersome, as all services that use
calendar timers would have to be modified that way, one by one. There is no way
to change machine's maintenance time easily, as was possible with cron.
Another way is to avoid the use of systemd timers and continue the use of cron.
Apart from this solution obviously going contrary to systemd spirit, it is
becoming harder to implement as distributions are beginning to migrate from
cron and will be removing it from base repositories and default installs.

Regression potential:
The regression potential of this change appears negligible, as we can keep the
default "00" semantics and change it only when new settings are used.

[1] http://comments.gmane.org/gmane.linux.arch.devel/20644
[2] https://mailman.archlinux.org/pipermail/arch-general/2014-April/036019.html

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