stephen_agent at jsw.gen.nz
Sat Sep 12 02:14:14 UTC 2020
On Fri, 11 Sep 2020 15:20:59 -0400, you wrote:
>After I get ACPI shutdown and wakeup working I'd like my database back-up
>to run right before shut down. Does anyone have a script written to do
>this, that they could share? TIA Daryl
Getting things to happen during shutdown is a fraught business. The
fundamental problem is that there are times when you need shutdown to
happen immediately, and anything that hinders that is a big problem.
For example, you have a hard drive that is making funny noises and you
need to get it shut down before you lose all its data. Or you have a
thunder storm passing over and you need to shut down and unplug all
your electronics before a nearby lightning strike kills them all. Or
you are rebooting your MythTV box to allow a kernel upgrade to happen,
and you have only 10 minutes between recordings do to that. Then the
shutdown backup runs and takes 10 minutes (mine does) and the next
recording starts before the shutdown happens.
In your case, you should be thinking of doing the database backup once
a day as usual, and preventing the initiation of an automatic shut
down while the backup is running, rather than starting the backup as
part of the shutdown process. So just let the backups happen as usual
as part of the anacron jobs in /etc/cron.weekly or /etc/cron.daily.
Then in your script that initiates shutdown, detect if the backup job
is running (or any anacron job is running) and wait for it to
complete. To detect if an anacron job is running, just check if the
program "anacron" is running. So I would add something like this to
the start of the shutdown script:
while pgrep anacron /dev/null 2>&1; do
# anacron is running - wait for a while.
If the normal time that anacron jobs are run is missed because the
system is shut down, the next time it wakes up, anacron will be run
automatically and the jobs will be run. Missed anacron jobs are run
shortly after the wakeup time - I think it is about a minute later.
The short delay is so as to not overload the system by running anacron
jobs at the same time as things that run when the PC wakes up.
In modern Ubuntu systems (since systemd), anacron is run by the
systemd units "anacron.service" and "anacron.timer". The
"anacron.timer" unit sets when it is run from a timer, and that unit
runs "anacron.service" every time the timer is triggered. The
"anacron.service" unit is run from the timer and usually also at
startup. You can configure how it all works. If you want
anacron.service to be run at startup, then you have to have the
anacron.service unit enabled:
sudo systemctl enable anacron
That is the default. If you do not want anacron.service to run at
startup, but only from the timer, disable anacron.service:
sudo systemctl disable anacron
Similarly, you can enable or disable anacron.timer:
sudo systemctl enable anacron.timer
sudo systemctl disable anacron.timer
For normal operation both units should be enabled, but for 24/7
servers, I prefer to disable anacron.service so that anacron jobs do
not get run immediately after a reboot that is done after midnight.
Maintenance on 24/7 servers is often done after midnight and it is a
big pain if anacron suddenly runs a whole set of jobs and you have to
wait for them to finish before you can reboot your server again!
However, anacron in Ubuntu has a bug with its default configuration.
When the change to systemd happened, whoever was responsible for
converting anacron to be run by systemd thought that anacron was
responsible for running the /etc/cron.hourly jobs, and therefore
anacron needed to be run every hour. In fact, the /etc/cron.hourly
jobs are run by cron, not anacron. But in Ubuntu 18.04 and earlier,
anacron.timer is set to run every hour, which is wrong and can cause
some significant complications. To fix that, I create a systemd
override file to run anacron.timer only once a day at a more normal
time like 05:30. To do that run this command:
sudo systemctl edit anacron.timer
That will open your default editor. Paste this text into the file:
Description=Trigger anacron at 05:30.
Save the file and exit, then run this:
sudo systemctl daemon-reload
Some time between Ubuntu 18.04 and Ubuntu 20.04, a change has been
made to anacron.timer to this:
Description=Trigger anacron every hour
Which I believe means that (despite the "Description" comment) anacron
will be run at 30 minutes past the hour, but only between 07:30 and
23:30. I am not sure why that was done, but it is also wrong and
still should be overridden as above.
More information about the mythtv-users