When I see status of sysstat is see the following,

● sysstat.service - Resets System Activity Logs
Loaded: loaded (/usr/lib/systemd/system/sysstat.service; enabled; 
vendor preset: enabled)
Active: active (exited) since Wed 2018-11-28 11:46:45 EST; 4s ago
Process: 4159 ExecStart=/usr/lib64/sa/sa1 --boot (code=exited, 
Main PID: 4159 (code=exited, status=0/SUCCESS)

Nov 28 11:46:45 localhost systemd[1]: Starting Resets System 
Activity Logs...
Nov 28 11:46:45 localhost systemd[1]: Started Resets System 
Activity Logs.

Also couldn’t find any pid specified above.

Question is, why this happens?

  • Deleted assumption as wrong assumption is pointless. – muhammad Nov 28 '18 at 20:32
  • Oneshot run, after which the regular performance capture is via cron, e.g. /etc/cron.d/sysstat – steve Nov 28 '18 at 22:32

Your assumption is wrong. sysstat is a "oneshot" type service, meaning it executes once and then it exits:

$ cat /usr/lib/systemd/system/sysstat.service

#... elided ...

ExecStart=/usr/lib64/sa/sa1 --boot

#... elided ...

... which explains why there is no PID -- because there is no process any more.

The documentation for systemd services says, in part:

Behavior of oneshot is similar to simple; however, it is expected that the process has to exit before systemd starts follow-up units.

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