On Wed Sep 11 10:45:48 PDT 2013 Lennart Poettering wrote:
So, it's a really bad idea to turn off UPS from userspace, simply
because you will race against the kernel's fs sync() code, and you never
know what will finish first: your UPS shutdown timeout or your fs
sync(). Doing this from userspace is hence racy.
If you want to use a hack like this then at least do it via the
/usr/lib/systemd/system-shutdown/ drop-in directory which is called very
late during shutdown. It will shorten the race window, but not erase it,
since the kernel will flush various buffers only after the reboot()
system call is invoked.
Here is the effect on a simple openSUSE 12.3 test rig of replacing my
home-made service unit /lib/systemd/system/ups-delayed-shutdown.service
with a script in the /usr/lib/systemd/system-shutdown/ drop-in directory.
I used the default UPS offdelay of 20 secs. The X's indicate the race
window. (Sorry for the ascii art.)
1) My "bad idea" ups-delayed-shutdown.service
systemctl system
poweroff halt
| |XXXXXXXXXXXXXXXX|
0 2 | 4 6 8 10 12 14 16 18 20 22 24 26 28 30 32 secs
| |
upsdrvctl UPS
shutdown shutdown
2) Replaced by script in /usr/lib/systemd/system-shutdown/
systemctl system
poweroff halt
| |XXXX|
0 2 4 6 8 |10 12 14 16 18 20 22 24 26 28 30 32 secs
| |
upsdrvctl UPS
shutdown shutdown
The script solution has the advantage of reducing the race window from 9
to 3 secs. Note that the same security can also be achieved by the
service unit by setting offdelay = 26 in ups.conf. My script is
#! /bin/bash
# Delayed turn off for the UPS unit.
# Needed for automatic system restart when wall power returns.
UPSDRVCTL_BIN=/usr/lib/ups/driver/upsdrvctl
$UPSDRVCTL_BIN shutdown
I tried adding
echo `date -I` `date +%T` "$0 calls $UPSDRVCTL_BIN shutdown" >>
/var/log/UPS.log
to get a trace of the activity, but nothing is written in
/var/log/UPS.log.
Roger
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